Fix the "menu permissions can be overridden by submenus" notice showing up for submenus 50% of the time.

Cause: `undefined` is not the same as `false`. $(...).data('submenu_id') returns undefined if the item does not have a submenu, which propagates all the way to $(notice).toggle(...). Calling toggle() with an undefined argument makes jQuery toggle the notice on/off regardless of whether the item has submenus.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@820991 b8457f37-d9ea-0310-8a92-e5e31aec5664