Simplified the implementation of register_model.

register_model is called exactly once in the entire Django code base, at the
bottom of ModelBase.__new__:

    new_class._meta.apps.register_model(new_class._meta.app_label, new_class)

ModelBase.__new__ exits prematurely 120 lines earlier (sigh) if a model with
the same name is already registered:

    if new_class._meta.apps.get_registered_model(new_class._meta.app_label, name):
        return

(This isn't the exact code, but it's equivalent.)

apps.register_model and apps.get_registered_model are essentially a setter and
a getter for apps.all_models, and apps.register_model is the only setter. As a
consequence, new_class._meta.apps.all_models cannot change in-between.

Considering that name == new_class.__name__, we can conclude that
register_model(app_label, model) is always called with such arguments that
get_registered_model(app_label, model.__name__) returns None.

Considering that model._meta.model_name == model.__name__.lower(), and looking
at the implementation of register_model and get_registered_model, this proves
that self.all_models[app_label] doesn't contain model._meta.model_name in
register_model, allowing us to simplify the implementation.