[SPARK-9385] [PYSPARK] Enable PEP8 but disable installing pylint.

[yhuai]

Instead of disabling all python style check, we should enable PEP8. So, this PR just comments out the part installing pylint.

[yhuai]

Alright, I saw

========================================================================
Running Python style checks
========================================================================
PEP8 checks passed.

in the jenkins output. I am merging it to master.

[SparkQA]

Test build #38596 has finished for PR 7704 at commit 0056359.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.