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 
Yeah, I would strongly suggest making it a one-time-token (if security is a criteria) since it is sent via email through an unencrypted channel.  But that is outside the scope of this PR.