fix(mqtt_cxx): Removes meaningless printf on subscribed handler

Removes the printf since the information isn't in the event.

Closes espressif#356

components/esp_mqtt_cxx/esp_mqtt_cxx.cpp

@@ -215,7 +215,6 @@ void Client::on_disconnected(esp_mqtt_event_handle_t const event)
 }
 void Client::on_subscribed(esp_mqtt_event_handle_t const event)
 {
-    printf("Subscribed to %.*s\r\n", event->topic_len, event->topic);
 }
 void Client::on_unsubscribed(esp_mqtt_event_handle_t const event)
 {