Make WebRtcAudioDeviceImpl::RemoveAudioRenderer not to change "playing_" flag.
Under current implementation, RemoveAudioRenderer/AddAudioRenderer can happen at any time without changing the current playout status. So it makes no sense to set 'playing_' flag to false in RemoveAudioRenderer. It's better to let only VoE manage the 'playing_' flag through Start/StopPlayout().
BUG=477434, 481532
TEST=Manual tests
R=tommi@chromium.org, henrika@chromium.org
Review URL: https://codereview.chromium.org/
1113063006
Cr-Commit-Position: refs/heads/master@{#328111}