nixos/nftables: Allow use with iptables (#121517)

* nixos/nftables: Allow use with iptables

Since iptables and nftables do not actually conflict with each other, there's no real reason to artificially prevent people from combining them.
In fact, this practice is known to cause issues like #88643, which is fixed by this commit.

nixos/modules/services/networking/nftables.nix

@@ -25,9 +25,10 @@ in
           for more information.
 
           There are other programs that use iptables internally too, such as
-          libvirt.
+          libvirt. For information on how the two firewalls interact, see [2].
 
           [1]: https://github.com/NixOS/nixpkgs/issues/24318#issuecomment-289216273
+          [2]: https://wiki.nftables.org/wiki-nftables/index.php/Troubleshooting#Question_4._How_do_nftables_and_iptables_interact_when_used_on_the_same_system.3F
         '';
     };
     networking.nftables.ruleset = mkOption {
@@ -118,20 +119,11 @@ in
           flush ruleset
           include "${cfg.rulesetFile}"
         '';
-        checkScript = pkgs.writeScript "nftables-check" ''
-          #! ${pkgs.runtimeShell} -e
-          if $(${pkgs.kmod}/bin/lsmod | grep -q ip_tables); then
-            echo "Unload ip_tables before using nftables!" 1>&2
-            exit 1
-          else
-            ${rulesScript}
-          fi
-        '';
       in {
         Type = "oneshot";
         RemainAfterExit = true;
-        ExecStart = checkScript;
-        ExecReload = checkScript;
+        ExecStart = rulesScript;
+        ExecReload = rulesScript;
         ExecStop = "${pkgs.nftables}/bin/nft flush ruleset";
       };
     };