removal of concrete Model from bases doesn't remove it from ModelState bases

[Sergey Fedoseev]

Steps to reproduce:

• add test app with models.py:

  class Thing(models.Model):
      pass
  
  
  class SuperThing(Thing):
      pass
  

• do

manage.py makemigrations test

• change models.py:

  class SuperThing(models.Model):
      thing_ptr = models.AutoField(primary_key=True)
  

• do

manage.py makemigrations test

• do it one more time

manage.py makemigrations test

last command results in

django.db.migrations.state.InvalidBasesError: Cannot resolve bases for [<ModelState: 'test.SuperThing'>]
This can happen if you are inheriting models from an app with migrations (e.g. contrib.auth)
 in an app with no migrations; see https://docs.djangoproject.com/en/1.7/topics/migrations/#dependencies for more