util: don't fallback to gnome-open

we should only use xdg-open directly, which will invoke gnome-open on Gnome system itself.
It doesn't make sense to only support gnome-open as a fallback, because some would ask
'are we liking gnome more than others?' or 'why aren't you using kde-open as a fallback?'

so just to save us from such question we should drop gnome-open support