I want remove last after wp_nav_menu

This will replace the last occurrence of your after string.

function replace_last_nav_item($items, $args) {
  return substr_replace($items, '', strrpos($items, $args->after), strlen($args->after));
}
add_filter('wp_nav_menu','replace_last_nav_item',100,2);

Pure CSS will do this too:

.btn_bar:last-child{display:none}