Revert "Add MTLIdentityValueTransformerName"

This reverts commit 8e80404b3dda92533991104737e31e1c544da80d.
Revert "Make transformer for key consistent with convenience methods"

This reverts commit 598d5caf96b9dc9de231fcc9b607f1c13fa11281.
Revert "WS"

This reverts commit 69ac21101a1243a635e8a1f7a855297b6df095f1.
Revert "Document non-nil requirement"

This reverts commit f3f62de1cb4a49bf0af76c1bb56fac96cbde4cb3.
Revert "Assert against nil transformers"

This reverts commit 1ca75a9d6c8b1816cc69146751b18b408d5b529e.
Handle nil from JSONTransformerForKey:

and entityAttributeTransformerForKey:, too