nixos/smokeping: Remove partOf to ensure restarts work.

Details on https://github.com/NixOS/nixops/issues/1063#issuecomment-453253666.

`partOf` makes that if `smokeping.service` is stopped, `thttpd.service` will
be stopped as well.
(But not that `thttpd` will be started when `smokeping` is started).

Once `thttpd.service` is stopped that way, `Restart = always` will not apply.

When the smokeping config options are changed, NixOS's `switch-configuration.pl`
will stop `smokeping` (whit shuts down thttpd due to `partOf`), and then restart
smokeping; but this does not start thttpd.
As a result, thttpd will be off after changing the config, which isn't desired.

This commit fixes it by removing the `partOf`, which makes `Restart` work
as expected.

nixos/modules/services/networking/smokeping.nix

@@ -332,7 +332,6 @@ in
     systemd.services.thttpd = mkIf cfg.webService {
       wantedBy = [ "multi-user.target"];
       requires = [ "smokeping.service"];
-      partOf = [ "smokeping.service"];
       path = with pkgs; [ bash rrdtool smokeping thttpd ];
       serviceConfig = {
         Restart = "always";