Remove an unused param from a CrasAudioHandler::AudioObserver method

This makes CrasAudioHandler::AudioObserver::OnOutputMuteChanged
match CrasAudioClient::Observer::OnOutputMuteChanged

Bug: 940810
Change-Id: If170df2f5c3cb451b4895d4052c39f42c25d8580
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1522552
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641655}
15 files changed