Remove _real_manager and _fallback_manager indirections

Those serve no real purpose: they only ever get used for getting a vanilla queryset out of them, which can be done instanciating the queryset directly. This is what their `get_queryset` method does anyway, and the classes are hardcoded so it is not even useful to change their behavior.

Let's go direct, pass the `shared_model` along to the queryset instead of creating a fake manager, only to have a custom property return the manager's model in the end.

This interface is already fully tested, this patch changes no behavior of it, so no need to add new tests.
This is internal implementation details, but just in case, we should add to the release notes that:
* `MyModel.objects._real_manager` should be replaced with `QuerySet(MyModel)`.
* `MyModel.objects._fallback_manager` should be replaced with `MyModel.objects.untranslated()`. 

Not sure why anyone would have used one of the formers though.