Remove FUNCTION_CLASS, in favor of a typeshed solution

jedi/evaluate/compiled/access.py

@@ -441,7 +441,6 @@ def _is_class_instance(obj):
 
 
 class _SPECIAL_OBJECTS(object):
-    FUNCTION_CLASS = types.FunctionType
     BOUND_METHOD_CLASS = type(DirectObjectAccess(None, None).py__bool__)
     MODULE_CLASS = types.ModuleType