[MINOR][DOC] Remove obsolete `ec2-scripts.md`

## What changes were proposed in this pull request?

Since this document became obsolete, we had better remove this for Apache Spark 2.3.0. The original document is removed via SPARK-12735 on January 2016, and currently it's just redirection page. The only reference in Apache Spark website will go directly to the destination in https://github.com/apache/spark-website/pull/54.

## How was this patch tested?

N/A. This is a removal of documentation.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #18578 from dongjoon-hyun/SPARK-REMOVE-EC2.

docs/ec2-scripts.md

----		
-layout: global
-title: Running Spark on EC2
-redirect: https://github.com/amplab/spark-ec2#readme
----
-
-This document has been superseded and replaced by documentation at https://github.com/amplab/spark-ec2#readme