Revert "Fixed #22645: Allow apps with no models module to still have migrations"

This reverts commit a4737bf6.

django/db/migrations/loader.py

-
 from __future__ import unicode_literals
 
 from importlib import import_module
@@ -63,6 +62,8 @@ class MigrationLoader(object):
         self.unmigrated_apps = set()
         self.migrated_apps = set()
         for app_config in apps.get_app_configs():
+            if app_config.models_module is None:
+                continue
             # Get the migrations module directory
             module_name = self.migrations_module(app_config.label)
             was_loaded = module_name in sys.modules