nixos/acme: remove unused binding in tests

  testCerts = import ./snakeoil-certs.nix;

  domain = testCerts.domain;

  resolver = let

    message = "You need to define a resolver for the acme test module.";

    firstNS = lib.head config.networking.nameservers;

  in if config.networking.nameservers == [] then throw message else firstNS;

  pebbleConf.pebble = {

    listenAddress = "0.0.0.0:443";

    managementListenAddress = "0.0.0.0:15000";