Deleting a field that uses a permission_callback does not delete the permission

Problem/Motivation

This may be the reason for, or a contributing factor to, the issue "After uninstalling the module the Anonymous and Authenticated user roles where deleted!"

Steps to reproduce

See the README for https://github.com/TolstoyDotCom/test_delete_field

Proposed resolution

TBD. Removing the permission on admin/people/permissions does work as expected.