Menu Customizer: don't add menu item data to menu item settings as the default value. Menu item settings are currently unused, so this theoretically avoids sending massive amounts of unused data back and forth to the server.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@947947 b8457f37-d9ea-0310-8a92-e5e31aec5664