Remove dead code

This code became dead when the builtin audio codec factories were
rewritten in https://codereview.webrtc.org/2997713002/.

BUG=webrtc:7821, webrtc:7822

Review-Url: https://codereview.webrtc.org/3003603002
Cr-Original-Commit-Position: refs/heads/master@{#19535}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d1d79f6866fcde94bb0354cec7d6ecaaf72de235
14 files changed