Delete iSAC-fb from NetEq

This is no longer used. Related code in the iSAC codec itself will be
deleted a follow-up CL.

BUG=4210

Review URL: https://codereview.webrtc.org/1404463003

Cr-Original-Commit-Position: refs/heads/master@{#10272}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 06b869f11a36c5abfb5e49a81ce7f88b2aa905d4
6 files changed