Clean up menu editing / creating

[filosofo]

Currently, wp_create_nav_menu is used only in wp_update_nav_menu_object.

• There's no need to expose a limited menu-creation API when wp_update_nav_menu_object handles that already.
• wp_create_nav_menu is doubling some of the sanity checks done already in wp_update_nav_menu_object, so we should simplify the logic.

wp_create_nav_menu() is already being used by plugins. I'd rather not take that away now that we are RC.

clean-up-menu-editing-api.13696.2.diff makes wp_create_nav_menu just wrap wp_update_nav_menu_object, so we still lose the duplicated logic.

[15104]