nixos/filesystems: remove unused variable (#384605)

      }

      {

        assertion = ! (any (fs: fs.formatOptions != null) fileSystems);

        message = let

          fs = head (filter (fs: fs.formatOptions != null) fileSystems);

        in ''

        message = ''

          'fileSystems.<name>.formatOptions' has been removed, since

          systemd-makefs does not support any way to provide formatting

          options.