Removed unneeded hint=None/obj=None in system check messages.

tests/invalid_models_tests/test_backend_specific.py

@@ -19,7 +19,7 @@ class BackendSpecificChecksTests(SimpleTestCase):
     @mock.patch('django.db.models.fields.router.allow_migrate', new=dummy_allow_migrate)
     def test_check_field(self):
         """ Test if backend specific checks are performed. """
-        error = Error('an error', hint=None)
+        error = Error('an error')
 
         class Model(models.Model):
             field = models.IntegerField()