[MINOR][CORE][TEST] Update obsolete `takeSample` test case.

## What changes were proposed in this pull request?

This PR fixes some obsolete comments and assertion in `takeSample` testcase of `RDDSuite.scala`.

## How was this patch tested?

This fixes the testcase only.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13260 from dongjoon-hyun/SPARK-15481.