[SPARK-14440][PYSPARK] Remove pipeline specific reader and writer

## What changes were proposed in this pull request?

https://issues.apache.org/jira/browse/SPARK-14440

Remove

* PipelineMLWriter
* PipelineMLReader
* PipelineModelMLWriter
* PipelineModelMLReader

and modify comments.

## How was this patch tested?

test with unit test.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #12216 from yinxusen/SPARK-14440.