[OTel] Remove unnecessary comment from codelab

examples/cpp/otel/codelab/greeter_callback_client_solution.cc

@@ -128,7 +128,6 @@ void RunClient(const std::string& target_str) {
 
 int main(int argc, char** argv) {
   absl::ParseCommandLine(argc, argv);
-  // CODELAB HINT : Add code to register OpenTelemetry plugin here.
   // Register a global gRPC OpenTelemetry plugin configured with a prometheus
   // exporter.
   opentelemetry::exporter::metrics::PrometheusExporterOptions opts;