fix(networking): disable IPv6 preference again

By default the system will already prefer IPv6 if possible without
causing issues e.g. with NAT.

I did indeed experience some issues with IPv6 requrests having a huge
delay using Tailscale.

The following article contains a detailed explanation about this issue:
https://www.ietf.org/archive/id/draft-buraglio-6man-rfc6724-update-03.html

system/networking.nix

@@ -36,13 +36,4 @@
   systemd.services.dnscrypt-proxy2.serviceConfig = {
     StateDirectory = lib.mkForce "dnscrypt-proxy2";
   };
-
-  # prefer IPv6
-  environment.etc."gai.conf".text = ''
-    label ::1/128       0
-    label ::/0          1
-    label 2002::/16     2
-    label ::/96         3
-    label ::ffff:0:0/96 4
-  '';
 }