APM: remove LevelEstimator

Only used in unit tests and a duplication of what `capture_output_rms_`
already does.

This CL also removes `AudioProcessingStats::output_rms_dbfs`, which is
now unused.

Bug: webrtc:5298
Fix: chromium:1261339
Change-Id: I6e583c11d4abb58444c440509a8495a7f5ebc589
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/235664
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35246}

modules/audio_processing/include/audio_processing.h

@@ -386,11 +386,6 @@ class RTC_EXPORT AudioProcessing : public rtc::RefCountInterface {
       bool enabled = true;
     } residual_echo_detector;
 
-    // Enables reporting of `output_rms_dbfs` in webrtc::AudioProcessingStats.
-    struct LevelEstimation {
-      bool enabled = false;
-    } level_estimation;
-
     std::string ToString() const;
   };