utils: Remove gnome-desktop-utils.[ch]

The single function it contained
(gnome_desktop_prepend_terminal_to_vector) is obsoleted by the
G_APP_INFO_CREATE_NEEDS_TERMINAL flag of GAppInfo.

The private _gnome_desktop_init_i18n() function is also unused,
its only caller having been removed.

https://bugzilla.gnome.org/show_bug.cgi?id=692385