MqttTransportHandler: Use default channelReadComplete implementation to avoid memory leaks.

common/transport/mqtt/src/main/java/org/thingsboard/server/transport/mqtt/MqttTransportHandler.java

@@ -421,11 +421,6 @@ public class MqttTransportHandler extends ChannelInboundHandlerAdapter implement
         return new MqttConnAckMessage(mqttFixedHeader, mqttConnAckVariableHeader);
     }
 
-    @Override
-    public void channelReadComplete(ChannelHandlerContext ctx) {
-        ctx.flush();
-    }
-
     @Override
     public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
         log.error("[{}] Unexpected Exception", sessionId, cause);