Removing enum that was left behind when the metric aec_quality_min was
removed.
Bug: webrtc:8563
Change-Id: I8a8c68659abc6465ef42f002f73bd2607e953ac5
Reviewed-on: https://webrtc-review.googlesource.com/33004
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21321}