nixos/prometheus-exporters: fix assertions declared in exporter modules

And it turns out, the test was using a removed option all along 🙃

nixos/modules/services/monitoring/prometheus/exporters.nix

@@ -230,6 +230,7 @@ let
     in
     mkIf conf.enable {
       warnings = conf.warnings or [];
+      assertions = conf.assertions or [];
       users.users."${name}-exporter" = (mkIf (conf.user == "${name}-exporter" && !enableDynamicUser) {
         description = "Prometheus ${name} exporter service user";
         isSystemUser = true;

nixos/tests/prometheus-exporters.nix

@@ -482,7 +482,6 @@ let
     json = {
       exporterConfig = {
         enable = true;
-        url = "http://localhost";
         configFile = pkgs.writeText "json-exporter-conf.json" (builtins.toJSON {
           modules = {
             default = {