Remove pub_topic and sub_topic class objects

There is no need to have the objects:

```c
std::string pub_topic[MQTT_TOPIC_MAX_SIZE_LIST];
std::string sub_topic[MQTT_TOPIC_MAX_SIZE_LIST];
```
Instead, just use the list std::list<mqtt_client_topic_data> subTopics; to store the `sub_topic` from the configs