models: remove obsolete method overrides

[ortholeviator]

From Django 1.7 release notes:

Schema migrations
-----------------

Django now has built-in support for schema migrations. It allows models
to be updated, changed, and deleted by creating migration files that represent
the model changes and which can be run on any development, staging or production
database.
Django 1.7 replaced South with its own migration framework.

From Django 1.10 release notes:

``Field.get_prep_lookup()`` and ``Field.get_db_prep_lookup()`` methods are removed
----------------------------------------------------------------------------------

If you have a custom field that implements either of these methods, register a
custom lookup for it. For example::

    from django.db.models import Field
    from django.db.models.lookups import Exact

    class MyField(Field):
        ...

    class MyFieldExact(Exact):
        def get_prep_lookup(self):
            # do_custom_stuff_for_myfield
            ....

    MyField.register_lookup(MyFieldExact)

[timabbott]

The South stuff should clearly be removed; what's your thinking on why we shouldn't replace the custom lookups with the new 1.10+ API?

[ortholeviator]

@timabbott django-bitfield already implemented two mechanisms for exposing the custom lookup class: the Field.get(_db)?_prep_lookup and RegisterLookupMixin.register_lookup. In Django 1.10+ they entirely removed the two functions, and therefore BitQueryLookupWrapper has been exclusively used for exact since then (an issue resolved together in #106).