boxes: Actually check for rectangle containment

Fixes condition duplicated:

      /* If a contains b, just remove b */
      if (meta_rectangle_contains_rect (a, b))
        {
          delete_me = other;
        }
      /* If b contains a, just remove a */
      else if (meta_rectangle_contains_rect (a, b))
        {
          delete_me = compare;
        }