Bug 1240423 - part6 : remove useless pref in Android. r=snorp

MozReview-Commit-ID: 8JFMZ5384wZ

--HG--
extra : rebase_source : 8d81b08eaac84181f63306a269f5e9c3478171b0

mobile/android/app/mobile.js

@@ -926,7 +926,3 @@ pref("dom.presentation.discovery.enabled", true);
 pref("dom.audiochannel.audioCompeting", true);
 // TODO : turn this pref default on in bug1264901
 pref("dom.audiochannel.mediaControl", false);
-
-// TODO : remove it after landing bug1242874 because now it's the only way to
-// suspend the MediaElement.
-pref("media.useAudioChannelAPI", true);