Nothing uses this now, so we can remove the dead code, and clean up the
API.
Since we're changing the shape of the return value anyway, we take the
opportunity to give the method a better name.
Adds a new method, check_3pid_auth, which gives password providers
the chance to allow authentication with third-party identifiers such
as email or msisdn.
Make sure that we delete devices whenever a user is logged out due to any of
the following situations:
* /logout
* /logout_all
* change password
* deactivate account (by the user or by an admin)
* invalidate access token from a dynamic module
Fixes#2672.
