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 
Users cannot currently subscribe to notifications on a per-course basis. It's either all on or all off. The token is actually an encryption of the username so that it can be used even if the user unsubscribes, resubscribes, and then uses an old link to unsubscribe again.