Bug fix: Direct method subscription not removed after disconnect

iothub/device/src/Pipeline/MqttTransportHandler.cs

@@ -1076,6 +1076,9 @@ private Task HandleDisconnectionAsync(MqttClientDisconnectedEventArgs disconnect
                 // During a disconnection, any pending twin updates won't be received, so we'll preemptively
                 // cancel these operations so the client can retry once reconnected.
                 RemoveOldOperations(TimeSpan.Zero);
+
+                _mqttClient.DisconnectedAsync -= HandleDisconnectionAsync;
+                _mqttClient.ApplicationMessageReceivedAsync -= HandleReceivedMessageAsync;
             }
 
             return Task.CompletedTask;