[SPARK-14418][PYSPARK] fix unpersist of Broadcast in Python

## What changes were proposed in this pull request?

Currently, Broaccast.unpersist() will remove the file of broadcast, which should be the behavior of destroy().

This PR added destroy() for Broadcast in Python, to match the sematics in Scala.

## How was this patch tested?

Added regression tests.

Author: Davies Liu <davies@databricks.com>

Closes #12189 from davies/py_unpersist.