[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.

This is a resubmission of @rxin's #1648 (closes #1648).
@rxin I'm not sure that it's safe to remove this synchronization around the `JobConf` constructor.  This synchronization was originally added to address HADOOP-10456, which affects a number of Hadoop versions that we still want to be compatible with.