[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.
Is it safe to re-assign to `self` here?  This seems to change the semantics of this `_reserialize()` method, since it now mutates the original RDD's serializer rather than returning a new one.  This might break / impact programs that aren't expecting an RDD object's serializer to change once that RDD is exposed to users (the reassignment to `converted._jrdd_deserializer` is safe because we haven't exposed `converted` to user code yet).