nixos/ups: remove Slice from UPS shutdown service

system-ups.slice conflicts with shutdown.target, so
ups-killpower.service is never queued at shutdown.
The slice isn't doing anything; removing it is a simple fix.

nixos/modules/services/monitoring/ups.nix

@@ -680,7 +680,6 @@ in
       serviceConfig = {
         Type = "oneshot";
         ExecStart = "${cfg.package}/bin/upsdrvctl shutdown";
-        Slice = "system-ups.slice";
       };
     };