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-Commit-Position: refs/heads/master@{#13316}
diff --git a/webrtc/modules/audio_processing/audio_processing_unittest.cc b/webrtc/modules/audio_processing/audio_processing_unittest.cc
index 23705e7..e5ab3da 100644
--- a/webrtc/modules/audio_processing/audio_processing_unittest.cc
+++ b/webrtc/modules/audio_processing/audio_processing_unittest.cc
@@ -1284,7 +1284,7 @@
   geometry.push_back(webrtc::Point(0.05f, 0.f, 0.f));
   config.Set<Beamforming>(new Beamforming(true, geometry));
   testing::NiceMock<MockNonlinearBeamformer>* beamformer =
-      new testing::NiceMock<MockNonlinearBeamformer>(geometry, 1u);
+      new testing::NiceMock<MockNonlinearBeamformer>(geometry);
   std::unique_ptr<AudioProcessing> apm(
       AudioProcessing::Create(config, beamformer));
   EXPECT_EQ(kNoErr, apm->gain_control()->Enable(true));