fix: devstack_docker -> devstack

In the context of edx-platform django settings,
devstack_docker is an alias to devstack. Both
are deprecated, technically, but we currently
want to remove all devstack-related settings files
except devstack.py (which tutor still uses). So,
in order to remove devstack_docker.py, we update
its references to devstack.py

docs/conf.py

@@ -320,10 +320,7 @@ def update_settings_module(service='lms'):
     Set the "DJANGO_SETTINGS_MODULE" environment variable appropriately
     for the module sphinx-apidoc is about to be run on.
     """
-    if os.environ.get('EDX_PLATFORM_SETTINGS') == 'devstack_docker':
-        settings_module = f'{service}.envs.devstack_docker'
-    else:
-        settings_module = f'{service}.envs.devstack'
+    settings_module = f'{service}.envs.devstack'
     os.environ['DJANGO_SETTINGS_MODULE'] = settings_module