drop now-unnecessary "or" clause on dataclass plugin init

Author: Bogdanp

mypy/plugin.py

@@ -311,8 +311,7 @@ def get_class_decorator_hook(self, fullname: str
                 attrs.attr_class_maker_callback,
                 auto_attribs_default=True
             )
-        # TODO: Drop the or clause once dataclasses lands in typeshed.
-        elif fullname in dataclasses.dataclass_makers or fullname.endswith('.dataclass'):
+        elif fullname in dataclasses.dataclass_makers:
             return dataclasses.dataclass_class_maker_callback
         return None