Consolidating the two approaches to GRPC non-jackson serialisable beans.
Consolidating the two approaches to GRPC non-jackson serialisable beans.

Summary:
- Deleting the more generic approach applied to all ObjectMappers (we can reintroduce if needed).
- Add comments to the MemoryHandler approach explained why it is needed.

Reviewed By: michsien

fbshipit-source-id: 10a225f456

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