[OTel] Remove unnecessary comment from codelab (#37403)

Closes #37403

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37403 from yashykt:OTelCodelab bd282f4459
PiperOrigin-RevId: 659738193

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;