Revert "Pull out the PostFilter to its own NonlinearBeamformer API"

This reverts commit b983112bc7686ed4276def4c9215c498444c6bdd.

It was breaking dependencies.
TBR=aluebs@webrtc.org
BUG=

Review URL: https://codereview.webrtc.org/2110503002 .

Cr-Original-Commit-Position: refs/heads/master@{#13316}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a3c51ea9f49872b9317e45c089b77f8187738dca
13 files changed