[SPARK-15433] [PYSPARK] PySpark core test should not use SerDe from PythonMLLibAPI

## What changes were proposed in this pull request?

Currently PySpark core test uses the `SerDe` from `PythonMLLibAPI` which includes many MLlib things. It should use `SerDeUtil` instead.

## How was this patch tested?
Existing tests.

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

Closes #13214 from viirya/pycore-use-serdeutil.