[SPARK-17828][DOCS] Remove unused generate-changelist.py

## What changes were proposed in this pull request?
We can remove this file based on discussion at https://issues.apache.org/jira/browse/SPARK-17828 it's evident this file has been redundant for a while, JIRA release notes serves this purpose for us already.

For ease of future reference you can find detailed release notes at, for example:

http://spark.apache.org/downloads.html -> http://spark.apache.org/releases/spark-release-2-0-1.html -> "Detailed changes" which links to https://issues.apache.org/jira/secure/ReleaseNote.jspa?projectId=12315420&version=12336857

## How was this patch tested?
Searched the codebase and saw nothing referencing this, hasn't been used in a while (probably manually invoked a long time ago)

Author: Adam Roberts <aroberts@uk.ibm.com>

Closes #15419 from a-roberts/patch-7.

dev/create-release/generate-changelist.py

-#!/usr/bin/python
-
-#
-# Licensed to the Apache Software Foundation (ASF) under one or more
-# contributor license agreements.  See the NOTICE file distributed with
-# this work for additional information regarding copyright ownership.
-# The ASF licenses this file to You under the Apache License, Version 2.0
-# (the "License"); you may not use this file except in compliance with
-# the License.  You may obtain a copy of the License at
-#
-#    http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-#
-# Creates CHANGES.txt from git history.
-#
-# Usage:
-#   First set the new release version and old CHANGES.txt version in this file.
-#   Make sure you have SPARK_HOME set.
-#   $  python generate-changelist.py
-
-
-import os
-import sys
-import subprocess
-import time
-import traceback
-
-SPARK_HOME = os.environ["SPARK_HOME"]
-NEW_RELEASE_VERSION = "1.0.0"
-PREV_RELEASE_GIT_TAG = "v0.9.1"
-
-CHANGELIST = "CHANGES.txt"
-OLD_CHANGELIST = "%s.old" % (CHANGELIST)
-NEW_CHANGELIST = "%s.new" % (CHANGELIST)
-TMP_CHANGELIST = "%s.tmp" % (CHANGELIST)
-
-# date before first PR in TLP Spark repo
-SPARK_REPO_CHANGE_DATE1 = time.strptime("2014-02-26", "%Y-%m-%d")
-# date after last PR in incubator Spark repo
-SPARK_REPO_CHANGE_DATE2 = time.strptime("2014-03-01", "%Y-%m-%d")
-# Threshold PR number that differentiates PRs to TLP
-# and incubator repos
-SPARK_REPO_PR_NUM_THRESH = 200
-
-LOG_FILE_NAME = "changes_%s" % time.strftime("%h_%m_%Y_%I_%M_%S")
-LOG_FILE = open(LOG_FILE_NAME, 'w')
-
-
-def run_cmd(cmd):
-    try:
-        print >> LOG_FILE, "Running command: %s" % cmd
-        output = subprocess.check_output(cmd, shell=True, stderr=LOG_FILE)
-        print >> LOG_FILE, "Output: %s" % output
-        return output
-    except:
-        traceback.print_exc()
-        cleanup()
-        sys.exit(1)
-
-
-def append_to_changelist(string):
-    with open(TMP_CHANGELIST, "a") as f:
-        print >> f, string
-
-
-def cleanup(ask=True):
-    if ask is True:
-        print "OK to delete temporary and log files? (y/N): "
-        response = raw_input()
-    if ask is False or (ask is True and response == "y"):
-        if os.path.isfile(TMP_CHANGELIST):
-            os.remove(TMP_CHANGELIST)
-        if os.path.isfile(OLD_CHANGELIST):
-            os.remove(OLD_CHANGELIST)
-        LOG_FILE.close()
-        os.remove(LOG_FILE_NAME)
-
-
-print "Generating new %s for Spark release %s" % (CHANGELIST, NEW_RELEASE_VERSION)
-os.chdir(SPARK_HOME)
-if os.path.isfile(TMP_CHANGELIST):
-    os.remove(TMP_CHANGELIST)
-if os.path.isfile(OLD_CHANGELIST):
-    os.remove(OLD_CHANGELIST)
-
-append_to_changelist("Spark Change Log")
-append_to_changelist("----------------")
-append_to_changelist("")
-append_to_changelist("Release %s" % NEW_RELEASE_VERSION)
-append_to_changelist("")
-
-print "Getting commits between tag %s and HEAD" % PREV_RELEASE_GIT_TAG
-hashes = run_cmd("git log %s..HEAD --pretty='%%h'" % PREV_RELEASE_GIT_TAG).split()
-
-print "Getting details of %s commits" % len(hashes)
-for h in hashes:
-    date = run_cmd("git log %s -1 --pretty='%%ad' --date=iso | head -1" % h).strip()
-    subject = run_cmd("git log %s -1 --pretty='%%s' | head -1" % h).strip()
-    body = run_cmd("git log %s -1 --pretty='%%b'" % h)
-    committer = run_cmd("git log %s -1 --pretty='%%cn <%%ce>' | head -1" % h).strip()
-    body_lines = body.split("\n")
-
-    if "Merge pull" in subject:
-        # Parse old format commit message
-        append_to_changelist("  %s %s" % (h, date))
-        append_to_changelist("  %s" % subject)
-        append_to_changelist("  [%s]" % body_lines[0])
-        append_to_changelist("")
-
-    elif "maven-release" not in subject:
-        # Parse new format commit message
-        # Get authors from commit message, committer otherwise
-        authors = [committer]
-        if "Author:" in body:
-            authors = [line.split(":")[1].strip() for line in body_lines if "Author:" in line]
-
-        # Generate GitHub PR URL for easy access if possible
-        github_url = ""
-        if "Closes #" in body:
-            pr_num = [line.split()[1].lstrip("#") for line in body_lines if "Closes #" in line][0]
-            github_url = "github.com/apache/spark/pull/%s" % pr_num
-            day = time.strptime(date.split()[0], "%Y-%m-%d")
-            if (day < SPARK_REPO_CHANGE_DATE1 or
-                (day < SPARK_REPO_CHANGE_DATE2 and pr_num < SPARK_REPO_PR_NUM_THRESH)):
-                github_url = "github.com/apache/incubator-spark/pull/%s" % pr_num
-
-        append_to_changelist("  %s" % subject)
-        append_to_changelist("  %s" % ', '.join(authors))
-        # for author in authors:
-        #     append_to_changelist("  %s" % author)
-        append_to_changelist("  %s" % date)
-        if len(github_url) > 0:
-            append_to_changelist("  Commit: %s, %s" % (h, github_url))
-        else:
-            append_to_changelist("  Commit: %s" % h)
-        append_to_changelist("")
-
-# Append old change list
-print "Appending changelist from tag %s" % PREV_RELEASE_GIT_TAG
-run_cmd("git show %s:%s | tail -n +3 >> %s" % (PREV_RELEASE_GIT_TAG, CHANGELIST, TMP_CHANGELIST))
-run_cmd("cp %s %s" % (TMP_CHANGELIST, NEW_CHANGELIST))
-print "New change list generated as %s" % NEW_CHANGELIST
-cleanup(False)