Revert "[SPARK-4882] Register PythonBroadcast with Kryo so that PySpark works with KryoSerializer"

This reverts commit 822a0b42f79acea2771d0b298e803c11c37aab81.

This fix does not apply to branch-1.1 or branch-1.0, since PythonBroadcast
is new in 1.2.