[SPARK-3886] [PySpark] simplify serializer, use AutoBatchedSerializer by default.

This PR simplify serializer, always use batched serializer (AutoBatchedSerializer as default), even batch size is 1.
With or without batching, it looks like the old code ended up flattening out all of the objects when writing them to a SequenceFile, so the end result / data was the same.  It looks like we already have other tests for reading Hadoop files, so I agree that this is probably safe to remove.