[SPARK-20637][CORE] Remove mention of old RDD classes from comments

## What changes were proposed in this pull request?

A few comments around the code mention RDD classes that do not exist anymore. I'm not sure of the best way to replace these, so I've just removed them here.

## How was this patch tested?

Only changes code comments, no testing required

Author: Michael Mior <mmior@uwaterloo.ca>

Closes #17900 from michaelmior/remove-old-rdds.