Checkpointable: Stop overriding __class__ in restore-on-create initializers

Seems to work fine without it, and the fact that it was returning a tuple was an issue (but I think is valid Python). These objects get unwrapped quickly anyway.

The odd thing is that this code has decent unit test coverage. Reproducing whatever isinstance caching in a unit test doesn't sound too helpful, since it'd basically be asserting that __class__ does not return a tuple.

PiperOrigin-RevId: 208671395