ibusManager: Don't use GNOME_SETUP_DISPLAY

GNOME_SETUP_DISPLAY is only necessary when using xwayland on demand,
which is only supported on systemd, in which case you should consider
using the user service provided by ibus.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2345>
(cherry picked from commit 4097f9f8)