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

Lib/test/test_typing.py

@@ -2921,8 +2921,8 @@ class ProtocolTests(BaseTestCase):
             def __init__(self):
                 self.x = None
 
-        self.assertIsInstance(C(), P)
-        self.assertIsInstance(D(), P)
+        self.assertIsInstance(CI(), P)
+        self.assertIsInstance(DI(), P)
 
     def test_protocols_in_unions(self):
         class P(Protocol):