This removes all the custom logic for deleting tables when using Django

1.7 and higher. While this was mostly working fine, there were edge

cases in the implementation, and they became problematic on Django 2.0+.

We now make use of the schema editor's logic to handle all model

deletions, allowing us to benefit from any changes made in Django going

forward.

Unit tests pass on Django 1.6 through 1.11. Along with other changes,

they also pass on 2.1.