Remove a bunch of now-unused stuff from djblets.auth.
Some of this hasn't been used in a long time, and some of it is only unused now

that I made changes to the Review Board code to use django.contrib.auth's

versions. I'm hoping to get rid of this package entirely, but registration

still has to be dealt with.

Ran djblets unit tests.
Ran Review Board unit tests.
Smoke tested authentication pages (log in, log out, user prefs, etc).