nixos/cupsd: stop managing /run/cups directory

The directory is already set up by cups.socket: managing it with
RuntimeDirectory in cups.service is unnecesary and has the unwanted
effect that upon stopping cupsd systemd will remove it. This includes
the /run/cups/cups.sock socket, so it breaks socket activation.

nixos/modules/services/printing/cupsd.nix

@@ -395,10 +395,7 @@ in
             ''}
           '';
 
-          serviceConfig = {
-            PrivateTmp = true;
-            RuntimeDirectory = [ "cups" ];
-          };
+          serviceConfig.PrivateTmp = true;
       };
 
     systemd.services.cups-browsed = mkIf avahiEnabled