[SPARK-15057][GRAPHX] Remove stale TODO comment for making `enum` in GraphGenerators

## What changes were proposed in this pull request?

This PR removes a stale TODO comment in `GraphGenerators.scala`

## How was this patch tested?

Just comment removed.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12839 from dongjoon-hyun/SPARK-15057.