code: void processUnsubscribe(ServerChannel session, String clientID, List<String> topics, int messageID) {
          Log.trace("processUnsubscribe invoked, removing subscription on topics {}, for clientID <{}>", topics, clientID);
  
          for (String topic : topics) {
              subscriptions.removeSubscription(topic, clientID);
          }
          //ack the client
          UnsubAckMessage ackMessage = new UnsubAckMessage();
          ackMessage.setMessageID(messageID);
  
          Log.trace("replying with UnsubAck to MSG ID {}", messageID);
          session.write(ackMessage);
      }
docs: Remove the clientID from topic subscription, if not previously subscribed,
  doesn't reply any error