services.datadog: remove python2 from systemd service (#228312)

which caused a security warning. seems to work without it so let's ditch it

nixos/modules/services/monitoring/datadog-agent.nix

···
 
     systemd.services = let
       makeService = attrs: recursiveUpdate {
-        path = [ datadogPkg pkgs.python pkgs.sysstat pkgs.procps pkgs.iproute2 ];
+        path = [ datadogPkg pkgs.sysstat pkgs.procps pkgs.iproute2 ];
         wantedBy = [ "multi-user.target" ];
         serviceConfig = {
           User = "datadog";