ui/utils: Don't allude to gnome-desktop's clock format

We're no longer trying to match gnome-desktop timestamps now, so drop the comment related to this in the ChronoExt code. Also rename the function from to_gnome_format() back to to_locale() as it was before (%x is locale-sensitive, and while the 12h/24h setting isn't strictly part of the locale, it's still strongly related).

Follow-up to 911b7353 ("archives_page: respect 12h/24h setting when displaying timestamps").