app: Stop using window visibility when comparing apps

shell_app_compare() (which is only used as sort function for
shell_app_system_get_running() nowadays) currently takes the
visibility of an app's windows into account, e.g. applications
with visible windows (non-minimized windows on current workspace)
sort earlier than applications without.
This translate traditional window-switcher behavior to applications,
but we stopped sorting by workspace in the app-switcher a while ago,
and with the new auto-minimization behavior of fullscreen windows
it is more confusing than helpful - in fact, since mutter commit
7e61ef09369a we no longer do this for the window list, so it
makes sense to apply the same to application sorting.

https://bugzilla.gnome.org/show_bug.cgi?id=707663
app: Removed unused function

https://bugzilla.gnome.org/show_bug.cgi?id=707663