FlowDescriptor is not removed from registry immediately when removing flow

At this time, on removeFlow action corresponding flow descriptor in DeviceFlowRegistry is marked as to-be-removed and then only when statistics are updated all marked flow descriptors are actually removed. This is not necessary since on stats-update all flows from tables are deleted and only actual are newly written (oper/DS). Also, flow descriptors won't be removed from registry in this way if statistics are turned off.