Misleading reference to UPower and missing error assignment

https://gitlab.xfce.org/xfce/xfce4-session/-/blob/54325662f3c00561411332506ead561d62c71e2b/xfce4-session/xfsm-logout-dialog.c#L302-392

xfsm_shutdown_can_suspend()
xfsm_shutdown_can_hibernate()
xfsm_shutdown_can_hybrid_sleep()

do not go through UPower anymore (since ages...).

The else branches are useless because xfsm_shutdown_can_{suspend,hibernate,hybrid_sleep} do not pass the error pointer to xfce_{consolekit,systemd}_can_* and never fail:

https://gitlab.xfce.org/xfce/xfce4-session/-/blob/54325662f3c00561411332506ead561d62c71e2b/xfce4-session/xfsm-shutdown.c#L542-664