Remove AudioProcessing::ChannelLayout

This enum is no longer needed. Also moving the last piece of code from
common.h to audio_processing_impl.h, allowing to delete common.h.

Bug: chromium:1271981, b/217349489
Change-Id: If115336c36d6d7b5845a903e421c18aebfe434ee
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251242
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35946}

modules/audio_processing/include/audio_processing.h

@@ -385,17 +385,6 @@ class RTC_EXPORT AudioProcessing : public rtc::RefCountInterface {
     std::string ToString() const;
   };
 
-  // TODO(mgraczyk): Remove once all methods that use ChannelLayout are gone.
-  enum ChannelLayout {
-    kMono,
-    // Left, right.
-    kStereo,
-    // Mono, keyboard, and mic.
-    kMonoAndKeyboard,
-    // Left, right, keyboard, and mic.
-    kStereoAndKeyboard
-  };
-
   // Specifies the properties of a setting to be passed to AudioProcessing at
   // runtime.
   class RuntimeSetting {