Revert "Reland Process 8 kHz audio as 16 kHz internally of the audio processing module"

This reverts commit b7b8e30cb44c41f51dbbefb9a9160e6dfe869c5b.

Reason for revert: Broke ApmTest.Process test in internal iOS waterfall

Original change's description:
> Reland Process 8 kHz audio as 16 kHz internally of the audio processing module
> 
> This CL relands the code from the CL "Process 8 kHz audio as 16 kHz internally
> of the audio processing module" which by mistake was reverted via a rebase in
> another CL.
> 
> The CL changes the behavior of APM for 8 kHz so that it is internally
> processed as 16 kHz.
> 
> Bug: webrtc:10863
> Change-Id: I32a57b2d279c2134125667c19b09cfda381a33c3
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150221
> Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
> Commit-Queue: Per Åhgren <peah@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#28944}

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

Change-Id: Ia49e07b0c25c49da646917516e317f1d57cc4e84
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:10863
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150326
Reviewed-by: Artem Titarenko <artit@webrtc.org>
Commit-Queue: Artem Titarenko <artit@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28948}

modules/audio_processing/include/audio_processing.h

@@ -698,7 +698,6 @@ class AudioProcessing : public rtc::RefCountInterface {
     kBadStreamParameterWarning = -13
   };
 
-  // Native rates supported by the AudioFrame interfaces.
   enum NativeRate {
     kSampleRate8kHz = 8000,
     kSampleRate16kHz = 16000,