nixos/systemd/network: use upstream default for privacy extensions

Privacy extensions were previously enabled through a udev rule, which
was then ported to networkd in [1]. But because networkd ships with
IPv6PrivacyExtensions=no by default[2], the udev rule used to get
overruled when the interface was managed through networkd.

The migration thereforce introduced a serious undocumented behavior
change by enabling IPv6 privacy extensions for all links managed by
networkd.

[1] bb954cddf5
[2] https://www.freedesktop.org/software/systemd/man/latest/systemd.network.html#IPv6PrivacyExtensions=

(cherry picked from commit 428bcb1fbe)

nixos/modules/system/boot/networkd.nix

@@ -2497,11 +2497,6 @@ let
   networkdOptions = {
     networkConfig = mkOption {
       default = { };
-      defaultText = lib.literalExpression ''
-        {
-          IPv6PrivacyExtensions = true;
-        }
-      '';
       example = {
         SpeedMeter = true;
         ManageForeignRoutingPolicyRules = false;
@@ -4001,10 +3996,7 @@ let
       };
 
       config = {
-        networkConfig = {
-          IPv6PrivacyExtensions = lib.mkOptionDefault true;
-        }
-        // optionalAttrs (config.routeTables != { }) {
+        networkConfig = optionalAttrs (config.routeTables != { }) {
           RouteTable = mapAttrsToList (name: number: "${name}:${toString number}") config.routeTables;
         };
       };