Fixed #34570 -- Silenced noop deferral of many-to-many and GFK.

While deferring many-to-many and GFK has no effect, the previous
implementation of QuerySet.defer() ignore them instead of crashing.

Regression in b3db6c8dcb.

Thanks Paco Martínez for the report.

tests/defer_regress/tests.py

@@ -271,6 +271,12 @@ class DeferRegressionTest(TestCase):
         with self.assertNumQueries(1):
             self.assertEqual(leaf.second_child.value, 64)
 
+    def test_defer_many_to_many_ignored(self):
+        location = Location.objects.create()
+        request = Request.objects.create(location=location)
+        with self.assertNumQueries(1):
+            self.assertEqual(Request.objects.defer("items").get(), request)
+
 
 class DeferDeletionSignalsTests(TestCase):
     senders = [Item, Proxy]