ref: Changes how isVideoPlayable is computed.

This commit changes how the SmallVideo.isVideoPlayable method works.

1st we remove the check on the video stream muted field (materialized with the
!this.videoStream.isMuted() guard). This check is redundant as it is
already materialized in the !this.isVideoMuted check (the isVideoMuted
field is updated with the return value of the videoStream.isMuted()
method).

2nd we return false if we're in audio only mode, because it's
(obviously) undesirable to have a playable video when in audio only
mode.

modules/UI/videolayout/SmallVideo.js

  * or <tt>false</tt> otherwise.
  */
 SmallVideo.prototype.isVideoPlayable = function() {
-    return this.videoStream && !this.isVideoMuted && !this.videoStream.isMuted();
+    return this.videoStream && !this.isVideoMuted && !APP.conference.isAudioOnly();
 };
 
 /**