Drop the 16kHz sample rate restriction on AECM and zero out higher bands

The restriction has been removed completely and AECM now supports any
number of higher bands. But this has been achieved by always zeroing out the
higher bands, instead of applying a constant gain which is the average over half
of the lower band (like it is done for the AEC), because that would be
non-trivial to implement and we don't want to spend too much time on AECM, since
we want to get rid of it in the long term anyway.

R=peah@webrtc.org, solenberg@webrtc.org, tina.legrand@webrtc.org

Review URL: https://codereview.webrtc.org/1774553002 .

Cr-Commit-Position: refs/heads/master@{#11931}

webrtc/modules/audio_processing/include/audio_processing.h

@@ -508,7 +508,6 @@ class AudioProcessing {
   static const int kNativeSampleRatesHz[];
   static const size_t kNumNativeSampleRates;
   static const int kMaxNativeSampleRateHz;
-  static const int kMaxAECMSampleRateHz;
 
   static const int kChunkSizeMs = 10;
 };