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}

webrtc/modules/audio_processing/include/audio_processing.h

@@ -31,7 +31,8 @@ struct AecCore;
 
 class AudioFrame;
 
-class NonlinearBeamformer;
+template<typename T>
+class Beamformer;
 
 class StreamConfig;
 class ProcessingConfig;
@@ -266,7 +267,7 @@ class AudioProcessing {
   static AudioProcessing* Create(const Config& config);
   // Only for testing.
   static AudioProcessing* Create(const Config& config,
-                                 NonlinearBeamformer* beamformer);
+                                 Beamformer<float>* beamformer);
   virtual ~AudioProcessing() {}
 
   // Initializes internal states, while retaining all user settings. This