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 
It is a requirement that old unsubscribe links continue to work indefinitely, so one-time-use tokens are not an option.