staging: xgifb: remove private ioctls

Drop the badly defined and broken private ioctl interface. Since the
driver is in staging, and some of the ioctls are clearly unsafe or not
even working, it's unlikely that there are any users.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>