dnscrypt-proxy module: serviceConfig.Group is redundant

Same as user's primary group if left unspecified

nixos/modules/services/networking/dnscrypt-proxy.nix

···
         ExecStart = "${dnscrypt-proxy}/bin/dnscrypt-proxy ${toString daemonArgs}";
 
         User = "dnscrypt-proxy";
-        Group = "dnscrypt-proxy";
 
         PrivateTmp = true;
         PrivateDevices = true;