Created row Type isn't deleted if whole model is deleted

[fjsj]

Steps to reproduce:

1. Remove whole model from models.py
2. makemigrations
3. migrate
4. migrate back to migration number before deletion
5. Error here:
   django.db.utils.ProgrammingError: type "restaurants_name_016c1c_zombodb_row_type" already exists

For now, solution is to make a custom database backend engine, like https://github.com/SectorLabs/django-postgres-extra. Then, in the custom backend, implement something like SpatialiteSchemaEditor.delete_model.

We won't fix this now since we want an easier to configure API and this is a low impact issue.