[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.

graphx/src/main/scala/org/apache/spark/graphx/util/GraphGenerators.scala

@@ -209,7 +209,6 @@ object GraphGenerators extends Logging {
     }
   }
 
-  // TODO(crankshaw) turn result into an enum (or case class for pattern matching}
   private def pickQuadrant(a: Double, b: Double, c: Double, d: Double): Int = {
     if (a + b + c + d != 1.0) {
       throw new IllegalArgumentException("R-MAT probability parameters sum to " + (a + b + c + d)