[examples/mqtt_no_blk] remove unused parameters

also change signature to `String&` instead of `char*` (as that is what is documented in the MQTT lib's README)

examples/MQTT/mqtt_basic_no_blk/mqtt_basic_no_blk.ino

@@ -47,7 +47,7 @@ IPAddress gateway(10, 10, 2, 2);  //Specify the gateway in case different from t
 IPAddress server(10, 10, 2, 2);   //The ip of the MQTT server
 char clientID[] = { "arduinoClient   " };
 
-void messageReceived(MQTTClient* client, char topic[], char payload[], int length) {
+void messageReceived(String& topic, String& payload) {
   Serial.println("incoming: ");
   Serial.print(topic);
   Serial.print(" - ");