Make _unregister_lookup be public API

[Andriy Sokolovskiy]

(as a follow-up for discussion from ​https://github.com/django/django/pull/4948)

Django allows users to register a custom lookups, but there are no public way to unregister it.
I don't see a reason to keep _unregister_lookup as a private API, so I'm proposing to make it be public and add documentation for it.

[Tim Graham]

Do you have a use case in mind?

[Andriy Sokolovskiy]

Well, my vision (in abstract) is if there is a public API to produce some changes (for this case to add new entity to existing collection) there should be a way to revert this change (for this case to remove entity from collection).

[Tim Graham]

We can wait for Anssi to say if there's any more explanation for making it private other than the existing comment, "Meant to be used in tests only," but I don't see much advantage to making a public method that doesn't have a concrete use case.

[Andriy Sokolovskiy]

Well, I prefer to remove it at all :) I don't see a reason to have method in codebase, which is used only in tests. We can avoid it as I described it in comment of my PR.
Simon's opinion is that this method is needed.
So, I think if we will leave it, it should be made public.

[Simon Charette]

My point is that I'd rather have a method (private or public) that deals with the implementation details of lookup unregistration than moving this logic in the tests by directly assigning to class_looukps.