modules: system: nix: don't change daemon niceness

This option doesn't really work the way it should anyway [1].

This reverts commit cbf6ea9ac9.

[1]: https://github.com/NixOS/nixpkgs/pull/138741

modules/system/nix/default.nix

@@ -18,9 +18,6 @@ in
         extraOptions = ''
           experimental-features = nix-command flakes
         '';
-
-        # Keep my system responsive during builds
-        daemonNiceLevel = 19;
       };
     }