Fix unused classes in a typing test (GH-102437)

As part of investigation issue https://github.com/python/cpython/issues/102433, I discovered what I believe to be an error where two classes `CI` and `DI` are not being used. The assertions beneath them act on `C` and `D`, duplicating existing assertions in this test.

Automerge-Triggered-By: GH:AlexWaygood