Always leave the _authentication_token set.

This was added a long, long time ago as a part of the StoreSessionModule in silverorange/store@64a2853 where CSRF prevention was added for all sessions that had accounts. It was then later moved to the SiteSessionModule in commit silverorange/site@1c9b22e. It was at this point that CSRF was added for all active sessions on the website. We want the ```_authentication_token``` set at all times now so don't unset it when a logout occurs.