[ticket/13069] Fix timezone location filtering.

$timezone contains the original jQuery object, therefore the subsequent
removal of the optgroup children while filtering has no effect on the
$replacement object - the one that's now visible to the user. We'll simplify
and fix this by injecting the content of the copy instead of going through the
trouble of cloning it.

[PHPBB3-13069](https://tracker.phpbb.com/browse/PHPBB3-13069)