Motivation:
 - we should never serialize xdr objects
 - some classes are composites of non Serializables, thus can't be
   serialized at all
 - SpotBugs generates a warning

Modification:
remove Serializable marker from autogenerated classes

Result:
cleaner code. No visible changes.

Acked-by: Olufemi Adeyemi
Acked-by: Lea Morschel
Target: master