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

[davies]

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.

[davies]

cc @yhuai

[SparkQA]

Test build #55053 has finished for PR 12189 at commit cdee210.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[holdenk]

Minor point: would it make sense to also update the PyDoc for unpersist to match the scaladoc while we are fixing its behavior anyways?

[yhuai]

Seems this PR does not change unpersist? Is the behavior of unpersist correct?

[davies]

@holdenk Updated.

@yhuai Changed unpersist(), the os.unlink() is moved to destroy()

[SparkQA]

Test build #55123 has finished for PR 12189 at commit e0408f3.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[yhuai]

LGTM. Merging to master. Thanks!