[1.6.x] Removed obsolete comment. Refs #20079.

Thanks Gavin Wahl.

django/contrib/auth/models.py

@@ -14,7 +14,6 @@
 from django.utils import timezone
 
 from django.contrib import auth
-# UNUSABLE_PASSWORD is still imported here for backwards compatibility
 from django.contrib.auth.hashers import (
     check_password, make_password, is_password_usable)
 from django.contrib.auth.signals import user_logged_in