AGC2: removed unused noise estimator implementation

This CL also includes the following changes:
- `AudioProcessing::Config::GainController2::noise_estimator`
  deprecated
- `EnergyToDbfs()` optimized by removing unnecessary `sqrt`
- Unit test minor fix, incorrect type was used

Bug: webrtc:7494
Change-Id: I88a6672d6f7cd03fcf6a3031883522d256880140
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/230940
Reviewed-by: Jesus de Vicente Pena <devicentepena@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34893}

modules/audio_processing/include/audio_processing.h

@@ -402,7 +402,6 @@ class RTC_EXPORT AudioProcessing : public rtc::RefCountInterface {
         bool enabled = false;
         // Run the adaptive digital controller but the signal is not modified.
         bool dry_run = false;
-        NoiseEstimator noise_estimator = kNoiseFloor;
         int vad_reset_period_ms = 1500;
         int adjacent_speech_frames_threshold = 12;
         float max_gain_change_db_per_second = 3.0f;
@@ -411,6 +410,7 @@ class RTC_EXPORT AudioProcessing : public rtc::RefCountInterface {
         bool avx2_allowed = true;
         bool neon_allowed = true;
         // TODO(crbug.com/webrtc/7494): Remove deprecated settings below.
+        NoiseEstimator noise_estimator = kNoiseFloor;
         float vad_probability_attack = 1.0f;
         LevelEstimator level_estimator = kRms;
         int level_estimator_adjacent_speech_frames_threshold = 12;