Add method for deleting menu items

Problem/Motivation

It would be nice to have a way to delete a menu item and optionally any children.

deleteMenuItem(string $menu_item, string $parent,  string $new_parent_for_children = NULL)

If $new_parent_for_children is NULL, then delete parent, otherwise, move the children to the new parent.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes