[SPARK-14154][MLLIB] Simplify the implementation for Kolmogorov–Smirnov test

## What changes were proposed in this pull request?
jira: https://issues.apache.org/jira/browse/SPARK-14154

I just read the code for KolmogorovSmirnovTest and find it could be much simplified following the original definition.

Send a PR for discussion

## How was this patch tested?
unit test

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #11954 from hhbyyh/ksoptimize.