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.