Update auth token

Don't unset the auth token on logout. This was causing every login
attempt, after a failed login attempt, to fail becuase the auth tokens
were different. Also remove code that is now done in the parent class.

To test:
1. Fail a login attempt to an admin.
2. Try and log in a second time. (On master this will fail silently no matter what credentials are provided)

See silverorange/site#39 for further details.