Delete legacy name AudioLevel in favor of the AudioLevelExtension

AudioLevel name was deprecated two weeks ago.

Bug: webrtc:15788
Change-Id: Idb26ab6ea701bcbceeda51640d521b78fa0d8162
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/341264
Auto-Submit: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41868}

modules/rtp_rtcp/source/rtp_header_extensions.h

diff --git a/modules/rtp_rtcp/source/rtp_header_extensions.h b/modules/rtp_rtcp/source/rtp_header_extensions.h
index 51c083a..7f039ec 100644
--- a/modules/rtp_rtcp/source/rtp_header_extensions.h
+++ b/modules/rtp_rtcp/source/rtp_header_extensions.h
@@ -100,9 +100,6 @@
                     bool voice_activity,
                     uint8_t audio_level);
 };
-// TODO: bugs.webrtc.org/15788 - In two weeks remove this alias and put new
-// meaning into AudioLevel class.
-using AudioLevel [[deprecated]] = AudioLevelExtension;
 
 class CsrcAudioLevel {
  public: