Also remove unused mixins. Bug: b/233192409 Change-Id: I959916b98cebd417088fb7804dc1f44f64134555 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/272241 Reviewed-by: Christoffer Jansson <jansson@google.com> Reviewed-by: Jeremy Leconte <jleconte@webrtc.org> Commit-Queue: Jeremy Leconte <jleconte@webrtc.org> Cr-Commit-Position: refs/heads/main@{#37826}