fix(large-video): clear remote video stream on track removal

VideoLayout schedules a large video update by passing in
the video stream on the small video instance. When a stream
is removed, the UI is removed from the small video instance
but a reference to the stream is left. So when VideoLayout
schedules the large video update after a stream removal,
the old stream from the small video instance is re-used,
even though it has been removed.

This change also brings balance with RemoteVideo method
"addRemoteStreamElement" which sets the stream on the
small video instance, so now "removeRemoteStreamElement
unsets it.

modules/UI/videolayout/RemoteVideo.js

@@ -357,6 +357,11 @@ RemoteVideo.prototype.removeRemoteStreamElement = function(stream) {
     logger.info(`${isVideo ? 'Video' : 'Audio'
     } removed ${this.id}`, select);
 
+
+    if (stream === this.videoStream) {
+        this.videoStream = null;
+    }
+
     this.updateView();
 };