Remove dependency on PHP 5.3

Raising the PHP-bar is not a nice thing. Lots of sites depend on 5.2.

You already mention this in below code. It that the only location for PHP5.3?

/**
 * @TODO Compatible with PHP 5.3 and more only. Find a way for 5.2 compatibility.
 */
function _taxonomy_menu_flatten_form_settings($settings) {
  $flatten = array();
  array_walk_recursive($settings, function($a, $b) use (&$flatten) { $flatten[$b] = $a; });

  return $flatten;
}