Add method for deleting a menu

Problem/Motivation

It would be nice to have a method to delete a menu (or at least delete all its menu_items) to make way for config sync to remove the menu.
Probably would not hurt to have it delete the menu too. If it was not removed from config sync it would just get recreated as empty, but this at least deletes it for people who do not use config sync.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes