Removed outdated comment of Notifier

Notifier is thread-hostile, and we have added a SequenceChecker
on https://webrtc-review.googlesource.com/c/src/+/252520 ,
so this comment is no longer needed.

Bug: None
Change-Id: I39f7f75a786dd27d2f6299d85676e7182d9032eb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/288180
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Daniel.L (Byoungchan) Lee <daniel.l@hpcnt.com>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38899}

sdk/android/src/jni/android_video_track_source.cc

@@ -65,8 +65,6 @@ void AndroidVideoTrackSource::SetState(JNIEnv* env,
     if (rtc::Thread::Current() == signaling_thread_) {
       FireOnChanged();
     } else {
-      // TODO(sakal): Is this even necessary, does FireOnChanged have to be
-      // called from signaling thread?
       signaling_thread_->PostTask([this] { FireOnChanged(); });
     }
   }