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 
Sorry, I misread your question. The key is the same, but value of the preference is different for each user. More specifically, the value of the preference is the token that should be used to generate the unsubscribe link.