secret service: don't be wanted by multi-user.target

This fixes an issue causing secret-trigger-*.service to be restarted on
every activation because after triggering these services are dead, this
in turn causes restarts of secret-*.service.

And finally this caused the issue of always restarting tinc services
as they are PartOf= a couple of secert-*.service.

krebs/3modules/secret.nix

@@ -27,7 +27,6 @@ in {
     systemd.services =
       mapAttrs'
         (name: file: nameValuePair "secret-trigger-${systemd.encodeName name}" {
-          wantedBy = ["multi-user.target"];
           serviceConfig = {
             Type = "oneshot";
             ExecStart = "${pkgs.systemd}/bin/systemctl restart ${shell.escape file.service}";