Revert "nixos/network-interfaces-systemd: fix IPv6 privacy extensions"

This reverts commit d349582c07.

The workaround initially applied isn't necessary anymore, as 247.3
contains the following commit:

> 242fc1d261 network: fix IPv6PrivacyExtensions=kernel handling

… which fixes https://github.com/systemd/systemd/issues/18003.

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -93,17 +93,7 @@ in
             (if i.useDHCP != null then i.useDHCP else false));
           address = forEach (interfaceIps i)
             (ip: "${ip.address}/${toString ip.prefixLength}");
-          # IPv6PrivacyExtensions=kernel seems to be broken with networkd.
-          # Instead of using IPv6PrivacyExtensions=kernel, configure it according to the value of
-          # `tempAddress`:
-          networkConfig.IPv6PrivacyExtensions = {
-            # generate temporary addresses and use them by default
-            "default" = true;
-            # generate temporary addresses but keep using the standard EUI-64 ones by default
-            "enabled" = "prefer-public";
-            # completely disable temporary addresses
-            "disabled" = false;
-          }.${i.tempAddress};
+          networkConfig.IPv6PrivacyExtensions = "kernel";
           linkConfig = optionalAttrs (i.macAddress != null) {
             MACAddress = i.macAddress;
           } // optionalAttrs (i.mtu != null) {