Revert "Refactoring of the noise suppressor and adding true multichannel support"

This reverts commit 87a7b82520b83a6cf42da27cdc46142c2eb6248c.

Reason for revert: Speculative revert. Breaks downstream projects.

Original change's description:
> Refactoring of the noise suppressor and adding true multichannel support
> 
> This CL adds proper multichannel support to the noise suppressor.
> To accomplish that in a safe way, a full refactoring of the noise
> suppressor code has been done.
> 
> Due to floating point precision, the changes made are not entirely
> bitexact. They are, however, very close to being bitexact.
> 
> As a safety measure, the former noise suppressor code is preserved
> and a kill-switch is added to allow revering to the legacy noise
> suppressor in case issues arise.
> 
> Bug: webrtc:10895, b/143344262
> Change-Id: I0b071011b23265ac12e6d4b3956499d122286657
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158407
> Commit-Queue: Per Åhgren <peah@webrtc.org>
> Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29646}

TBR=gustaf@webrtc.org,peah@webrtc.org

Change-Id: I4d4025bda01f484979961fe57380a705e4d78397
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:10895, b/143344262
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158701
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29651}

modules/audio_processing/include/audio_processing.h

@@ -285,8 +285,6 @@ class RTC_EXPORT AudioProcessing : public rtc::RefCountInterface {
       bool enabled = false;
       enum Level { kLow, kModerate, kHigh, kVeryHigh };
       Level level = kModerate;
-      // Recommended not to use. Will be removed in the future.
-      bool use_legacy_ns = false;
     } noise_suppression;
 
     // Enables reporting of |voice_detected| in webrtc::AudioProcessingStats.