Removed unused GetOutputVolume() and SetOutputVolume() from MediaEngineInterface.

BUG=webrtc:4690

Review-Url: https://codereview.webrtc.org/2059403002
Cr-Original-Commit-Position: refs/heads/master@{#13135}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 05b9803c8ef344ac3dc8fb59504b87a227a0d39d
7 files changed