Part of [SPARK-2456] Removed some HashMaps from DAGScheduler by storing information in Stage.

This is part of the scheduler cleanup/refactoring effort to make the scheduler code easier to maintain. 

@kayousterhout @markhamstra please take a look ... 
Great question...I think we just didn't notice that.  I think it's safe to remove it, and you can also remove it as a parameter to failJobAndIndependentStages