Remove unnecessary UsernameCipher code

UsernameCipher was copied from edx-platform, but it is not necessary
because the encrypted token needed for the unsubscribe URL is contained
in the user's preference that is also included in the payload from the
LMS user API.

@jimabramson @nasthagiri 
possible to avoid the hardcoded host/port in the test?  (unless it's hardcoded as a constant somewhere in which case i think its fine)