stdlib: Remove duplicated Pickler.reducer_override()

The declaration was present twice. One from #3358 with the bulk Python
3.8 support, the other from #3636 with Pickle protocol 5 support.

Author: moreati

stdlib/2and3/pickle.pyi

@@ -82,8 +82,6 @@ class Pickler:
     def dump(self, obj: Any) -> None: ...
     def clear_memo(self) -> None: ...
     def persistent_id(self, obj: Any) -> Any: ...
-    if sys.version_info >= (3, 8):
-        def reducer_override(self, obj: Any) -> Any: ...
 
 class Unpickler:
     if sys.version_info >= (3, 8):