[SPARK-2585] Remove special handling of Hadoop JobConf.

Previously we broadcast JobConf for HadoopRDD because it is large. Now we always broadcast RDDs and task closures so it should no longer be necessary to broadcast the JobConf anymore.
It might be worth a comment here then saying that the createJobConf() method really does create a new job conf because <xyz> even though it looks like it's just accessing the broadcast value.