Remove VoENetEqStats interface.

(TBR stefan@ for changes to webrtc/test/mock_voice_engine.h)

BUG=webrtc:4690
TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/2744953003
Cr-Original-Commit-Position: refs/heads/master@{#17201}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c6192a9e3207cd3c2815939c9c6a68cbf97905de