[SPARK-14157][SQL] Parse Drop Function DDL command

## What changes were proposed in this pull request?
JIRA: https://issues.apache.org/jira/browse/SPARK-14157

We only parse create function command. In order to support native drop function command, we need to parse it too.

From Hive [manual](https://cwiki.apache.org/confluence/display/Hive/LanguageManual+DDL#LanguageManualDDL-Create/Drop/ReloadFunction), the drop function command has syntax as:

DROP [TEMPORARY] FUNCTION [IF EXISTS] function_name;

## How was this patch tested?

Added test into `DDLCommandSuite`.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #11959 from viirya/parse-drop-func.