Delete iSAC-fb from AudioCodingModule

This is no longer used. Related code in NetEq and the iSAC codec itself
will be deleted in follow-up CLs.

BUG=4210

Review URL: https://codereview.webrtc.org/1404623002

Cr-Original-Commit-Position: refs/heads/master@{#10264}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9ea2147e5cbde3fd71a3c20353494a7e356ac98a
8 files changed