[manual] Remove now-unused User model.

I think all that one needs to do to deploy this commit is on developer
laptops, run `generate-fixtures --force`.

(imported from commit 34916341435fef0875b5a2c7f53c2f5606cd16cd)

zephyr/tests.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 
-from django.contrib.auth.models import User
 from django.test import TestCase
 from django.test.simple import DjangoTestSuiteRunner
 from django.utils.timezone import now