Remove unused VoiceChannelTransport.

BUG=webrtc:4690

Review-Url: https://codereview.webrtc.org/2687843002
Cr-Original-Commit-Position: refs/heads/master@{#17086}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 759358c9dbc47b22e8e821d5e4cca81d2dcff44f
23 files changed