In `wp-admin/includes/nav-menu.php`, remove dead code:

* In `_wp_ajax_menu_quick_search()`, in an `else` block, `$post_obj` was set and not used. Another block sets it again and does use it. 
* In `wp_nav_menu_item_post_type_meta_box()`, `$post_type_object` was set and, sadly, never used. `$error` was set and not used. 

See #27882.


Built from https://develop.svn.wordpress.org/trunk@28300


git-svn-id: http://core.svn.wordpress.org/trunk@28128 1a063a9b-81f0-0310-95a4-ce76da25c4cd