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.gypi b/webrtc/modules/audio_processing/audio_processing.gypi
index 65a79a9..4ce67da 100644
--- a/webrtc/modules/audio_processing/audio_processing.gypi
+++ b/webrtc/modules/audio_processing/audio_processing.gypi
@@ -66,6 +66,7 @@
         'audio_processing_impl.h',
         'beamformer/array_util.cc',
         'beamformer/array_util.h',
+        'beamformer/beamformer.h',
         'beamformer/complex_matrix.h',
         'beamformer/covariance_matrix_generator.cc',
         'beamformer/covariance_matrix_generator.h',