Fix up warnings

- the default auto-increment primary key field type is now
  configurable, and Django's check command issues a warning if you are
  just assuming the historical behavior of using AutoField.

- Django 3.2 brings in automatic AppConfig discovery, so all of our
  explicit `default_app_config = ...` assignments in __init__.py
  modules are no longer needed, and raise a RemovedInDjango41Warning.

awx/conf/__init__.py

@@ -7,8 +7,6 @@ from django.utils.module_loading import autodiscover_modules
 # AWX
 from .registry import settings_registry
 
-default_app_config = 'awx.conf.apps.ConfConfig'
-
 
 def register(setting, **kwargs):
     settings_registry.register(setting, **kwargs)