Revert "Update path in udev rule of mdadm"

This reverts commit c9364fdbb7.

See https://github.com/NixOS/nixpkgs/pull/6006

pkgs/os-specific/linux/mdadm/default.nix

@@ -22,12 +22,6 @@ stdenv.mkDerivation rec {
       fi
     '';
 
-  preInstall =
-    ''
-    substituteInPlace udev-md-raid-assembly.rules --replace "/sbin/mdadm" "$out/sbin/mdadm"
-    substituteInPlace udev-md-raid-arrays.rules --replace "/sbin/mdadm" "$out/sbin/mdadm"
-    '';
-
   meta = {
     description = "Programs for managing RAID arrays under Linux";
     homepage = http://neil.brown.name/blog/mdadm;