[SPARK-2079] Removing unnecessary wrapping in SchemaRDD.javaToPython

@ahirreddy , you are right; batching is currently not implemented in ```javaToPython``` method. We could change it to ```BatchedSerializer(PickleSerialzier())``` later when batching is supported. Also, ```javaToPython``` only serializes to Pickle format for now, while ```_sc.serializer``` could potentially be set to some other serializers (like ```MarshalSerializer```) when user creates ```_sc```.