Revert "Fixed #16865 -- Made get_or_create use read database for initial get query."

Thanks to Jeremy Dunck for pointing out the problem with this change. If in a
single transaction, the master deletes a record and then get_or_creates a
similar record, under the new behavior the get_or_create would find the record
in the slave db and fail to re-create it, leaving the record nonexistent, which
violates the contract of get_or_create that the record should always exist
afterwards. We need to do everything against the master here in order to ensure
correctness.

This reverts commit 901af86550.

tests/modeltests/get_or_create/tests.py

@@ -64,27 +64,3 @@ class GetOrCreateTests(TestCase):
             formatted_traceback = traceback.format_exc()
             self.assertIn('obj.save', formatted_traceback)
 
-
-    def test_initial_get_on_read_db(self):
-        """
-        get_or_create should only set _for_write when it's actually doing a
-        create action. This makes sure that the initial .get() will be able to
-        use a slave database. Specially when some form of database pinning is
-        in place this will help to not put all the SELECT queries on the
-        master. Refs #16865.
-
-        """
-        qs = Person.objects.get_query_set()
-        p, created = qs.get_or_create(
-            first_name="Stuart", last_name="Sutcliffe", defaults={
-                "birthday": date(1940, 6, 23),
-                }
-            )
-        self.assertTrue(created)
-        self.assertTrue(qs._for_write)
-
-        qs = Person.objects.get_query_set()
-        p, created = qs.get_or_create(
-            first_name="Stuart", last_name="Sutcliffe")
-        self.assertFalse(created)
-        self.assertFalse(qs._for_write)