Remove the only_with_models_module argument of get_app_config[s]

[Aymeric Augustin]

Once #21676 is done, the current logic that calls either populate_apps or populate_models won't make sense any more.

At this point it would be more straightforward to remove this argument and let callers filter the results by themselves.

[Aymeric Augustin <aymeric.augustin@…>]

In bfcc686d22df10c268752635947dd242317ba156:

Removed the only_with_models_module argument of get_model[s].

Now that the refactorings are complete, it isn't particularly useful any
more, nor very well named. Let's keep the API as simple as possible.

Fixed #21689.