Use std::variant instead of mapbox::util::variant (#6903)

src/benchmarks/route.cpp

@@ -76,7 +76,7 @@ try
         {
             engine::api::ResultT result = json::Object();
             const auto rc = osrm.Route(params, result);
-            auto &json_result = result.get<json::Object>();
+            auto &json_result = std::get<json::Object>(result);
             if (rc != Status::Ok || json_result.values.find("routes") == json_result.values.end())
             {
                 throw std::runtime_error{"Couldn't route"};