Bug 1580659 - part2 : remove redundant checking. r=chunmin

We have already had the same checking in `AudioCaptureStreamChange()`, so we can remove the checking in `AudioCaptureStreamChangeIfNeeded()`.

Differential Revision: https://phabricator.services.mozilla.com/D45749

dom/html/HTMLMediaElement.cpp

@@ -1107,11 +1107,6 @@ class HTMLMediaElement::AudioChannelAgentCallback final
     if (!IsPlayingStarted()) {
       return;
     }
-
-    if (!mOwner->HasAudio()) {
-      return;
-    }
-
     mOwner->AudioCaptureStreamChange(mAudioCapturedByWindow);
   }