Clean up user app from MA artifacts

[akatsoulas]

After #2161 is resolved, we need to cleanup any flags from the database that check if the user is migrated to MA and clean up any code related to authentication that refers to old sumo accounts.

[smithellis]

mozilla/kitsune#6596

[smithellis]

I believe I covered all the cases, but left this as WIP pending some feedback.

[smithellis]

PR ready and approved, but can't merge until other MA items are closed.