LibWeb: Replace MarkedVector with Vector in AudioTrackList

There's no need to make the m_audio_tracks entries heap roots, we can
visit them instead.

Userland/Libraries/LibWeb/HTML/VideoTrackList.cpp

@@ -17,7 +17,6 @@ JS_DEFINE_ALLOCATOR(VideoTrackList);
 
 VideoTrackList::VideoTrackList(JS::Realm& realm)
     : DOM::EventTarget(realm, MayInterfereWithIndexedPropertyAccess::Yes)
-    , m_video_tracks(realm.heap())
 {
 }