remove unnecessary __str__ in test

Author: bckohan

django_enum/tests/enum_prop/enums.py

@@ -107,9 +107,6 @@ class ExternEnum(IntEnumProperties, s('label', case_fold=True)):
         TWO   = 2, 'Two'
         THREE = 3, 'Three'
 
-        def __str__(self):
-            return self.label
-
 
 except (ImportError, ModuleNotFoundError):  # pragma: no cover
     pass