Menus: Remove unused commented out code in `wp-admin/nav-menus.php`.

Props garrett-eclipse, audrasjb.
Fixes #49967.
Built from https://develop.svn.wordpress.org/trunk@47605


git-svn-id: https://core.svn.wordpress.org/trunk@47380 1a063a9b-81f0-0310-95a4-ce76da25c4cd