test_tf_common: remove un_used mixin class parameters (#6866)

tests/test_modeling_tf_xlnet.py

@@ -348,7 +348,6 @@ class TFXLNetModelTest(TFModelTesterMixin, unittest.TestCase):
     all_generative_model_classes = (
         (TFXLNetLMHeadModel,) if is_tf_available() else ()
     )  # TODO (PVP): Check other models whether language generation is also applicable
-    test_pruning = False
 
     def setUp(self):
         self.model_tester = TFXLNetModelTester(self)