[SPARK-4802] [streaming] Remove receiverInfo once receiver is de-registered

  Once the streaming receiver is de-registered at executor, the `ReceiverTrackerActor` needs to
remove the corresponding reveiverInfo from the `receiverInfo` map at `ReceiverTracker`.
It seems a little suspicious to me that we assign to `receiverInfo` on this line only to remove the value that we set two lines later.  I think it would be clearer to remove this line and change the next line to read

```scala
listenerBus.post(StreamingListenerReceiverStopped(newReceiverInfo))
```

instead.