Remove `isinstance(user, AnonymousUser)` checks

Turns out `ESPUser` inherits from `AnonymousUser`, so that doesn't do what one
might think.  Generally replaced by `user.is_anonymous()` which does work
correctly on ESPUsers.  I didn't remove it from `ESPAuthMiddleware` because I
don't understand that well enough to know if I will break something massively.
Also removed an unused function rather than fixing it, and some unused imports
of `AnonymousUser`.
Merge pull request #1518 from learning-unlimited/anonymous-user-fix

Remove `isinstance(user, AnonymousUser)` checks