Remove apparently obsolete FilterMixin metaclass

[abbottc]

There is a metaclass on FilterMixin that was marked to be removed in version 2.1, but still exists: https://github.com/carltongibson/django-filter/blob/ab7978323ff092c0ccd2b95e9b190ba8522bde11/django_filters/views.py#L13

The metaclass looks like it only serves to give a warning if the long-since deprecated name filter_fields is used.

In case this was forgotten, I just wanted to bump its removal, since subclasses of FilterMixin that need their own metaclasses must do a little bit of working around this apparently obsolete metaclass as long as it still exists.

If it wasn't forgotten but remains there intentionally, is it planned for removal at any specific later version?

[carltongibson]

Hey @abbottc -- Yes, there's a couple of these cleanups lingering i think. Would you fancy taking them on?

[abbottc]

Sure I can take care of them. What else needs to be cleaned up? Or are all of them marked with a clear TODO like this one that would make them easy to search for?

[carltongibson]

A quick grep for remove suggests at least a good bunch are marked with the TODO. (Didn't spot any others, but it was a very quick grep 😀)

Thanks!