Remove dead code (we no longer support SILK)

Review URL: https://codereview.webrtc.org/1461043002

Cr-Original-Commit-Position: refs/heads/master@{#10715}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 12e21a0d6ce70b86ffafec10a5004ef2b1826dba
2 files changed