[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`.

Author: Ilayaperumal Gopinathan <igopinathan@pivotal.io>

Closes #3647 from ilayaperumalg/receiverInfo-RTracker and squashes the following commits:

6eb97d5 [Ilayaperumal Gopinathan] Polishing based on the review
3640c86 [Ilayaperumal Gopinathan] Remove receiverInfo once receiver is de-registered

(cherry picked from commit 10d69e9cbfdabe95d0e513176d5347d7b59da0ee)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>

Conflicts:
	streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceiverTracker.scala