Don't return ``user_name`` in mapped.Mapped class.

auth.mapped.Mapped._handle_unscoped_token() and _setup_username don't
need to return and store variable ``user_name``. Drop it.

Change-Id: Id66e63a49a146a04217ad5c548cd877819aa9ca2
Merge "Don't return ``user_name`` in mapped.Mapped class."