[openbsd] support for running shutdown as user being in operator group is being dropped

added in 1d56462b, but that ability might soon be removed, only leaving us with the ability to shutdown via ckit2/polkit (or manual launchers with doas halt -p)

if that move is made, i'll make sure to make it return false in https://gitlab.xfce.org/xfce/xfce4-session/-/blob/master/xfce4-session/xfsm-shutdown-fallback.c#L273