nixos/systemd-confinement: remove unused rootName

nixos/modules/security/systemd-confinement.nix

···
       };
 
       config = let
-        rootName = "${mkPathSafeName name}-chroot";
         inherit (config.confinement) binSh fullUnit;
         wantsAPIVFS = lib.mkDefault (config.confinement.mode == "full-apivfs");
       in lib.mkIf config.confinement.enable {