Create a `delete_user` function

[jjnesbitt]

We current have a register_user function, but we don't have one to delete a user. If nothing else, this would be useful for testing, as well as thoroughness of our API.

[waxlamp]

We will likely want that delete_user function when we have a need for a managed_user test fixture. It would also be useful for GDPR/CCPA-type compliance.