[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`.
Yeah, that's essentially what I was proposing.  I was suggesting

```scala
listenerBus.post(StreamingListenerReceiverStopped(newReceiverInfo))
receiverInfo.remove(streamId)
```

but I don't think there's going to be a huge different between this and your suggestion unless there are multi-threading concerns.  Feel free to update this with your suggestion.