Bug 1196361 - Remove the media.useAudioChannelService pref; r=baku

We need to rely on the audio channel service for all products now,
so supporting running without it seems pointless at this point.

dom/html/HTMLMediaElement.cpp

@@ -1097,11 +1097,6 @@ static bool IsAutoplayEnabled()
   return Preferences::GetBool("media.autoplay.enabled");
 }
 
-static bool UseAudioChannelService()
-{
-  return Preferences::GetBool("media.useAudioChannelService");
-}
-
 static bool UseAudioChannelAPI()
 {
   return Preferences::GetBool("media.useAudioChannelAPI");
@@ -2411,10 +2406,6 @@ bool HTMLMediaElement::ParseAttribute(int32_t aNamespaceID,
 
 bool HTMLMediaElement::CheckAudioChannelPermissions(const nsAString& aString)
 {
-  if (!UseAudioChannelService()) {
-    return true;
-  }
-
   // Only normal channel doesn't need permission.
   if (aString.EqualsASCII("normal")) {
     return true;
@@ -4457,10 +4448,6 @@ ImageContainer* HTMLMediaElement::GetImageContainer()
 
 nsresult HTMLMediaElement::UpdateChannelMuteState(float aVolume, bool aMuted)
 {
-  if (!UseAudioChannelService()) {
-    return NS_OK;
-  }
-
   if (mAudioChannelVolume != aVolume) {
     mAudioChannelVolume = aVolume;
     SetVolumeInternal();
@@ -4487,10 +4474,6 @@ nsresult HTMLMediaElement::UpdateChannelMuteState(float aVolume, bool aMuted)
 
 void HTMLMediaElement::UpdateAudioChannelPlayingState()
 {
-  if (!UseAudioChannelService()) {
-    return;
-  }
-
   bool playingThroughTheAudioChannel =
      (!mPaused &&
       !Muted() &&