nixos/stage-1-systemd: fix initrd-fstab generation for bind mounts, again

See https://github.com/NixOS/nixpkgs/pull/185089

nixos/modules/tasks/filesystems.nix

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       # https://wiki.archlinux.org/index.php/fstab#Filepath_spaces
       escape = string: builtins.replaceStrings [ " " "\t" ] [ "\\040" "\\011" ] string;
     in fstabFileSystems: { rootPrefix ? "" }: concatMapStrings (fs:
-      (optionalString (isBindMount fs) (escape rootPrefix))
-      + (if fs.device != null then escape fs.device
+      (if fs.device != null then escape fs.device
          else if fs.label != null then "/dev/disk/by-label/${escape fs.label}"
          else throw "No device specified for mount point ‘${fs.mountPoint}’.")
       + " " + escape fs.mountPoint