Remove unused import statements (#2282)

third_party/3/enum.pyi

@@ -1,6 +1,6 @@
 # NB: third_party/3/enum.pyi and stdlib/3.4/enum.pyi must remain consistent!
 import sys
-from typing import List, Any, TypeVar, Union, Iterator, TypeVar, Generic, Type, Sized, Mapping
+from typing import Any, Iterator, List, Mapping, Type, TypeVar, Union
 from abc import ABCMeta
 
 _T = TypeVar('_T')