Remove unused code in APM

- The injection of the AGC2 level estimator into `AgcManagerDirect`
  is not used anymore
- `ExperimentalAgc::enabled_agc2_level_estimator` can also be removed
- 3 ctors of `ExperimentalAgc` are unused
- `AgcManagerDirectStandaloneTest::AgcMinMicLevelExperiment` can be
  split into separate unit tests (better code clarity)

Bug: webrtc:7494
Change-Id: I5843147c38cf7cb5ee484b0a72fe13dcf363efaf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/202025
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33027}

modules/audio_processing/test/audio_processing_simulator.h

@@ -106,7 +106,6 @@ struct SimulationSettings {
   absl::optional<bool> use_vad;
   absl::optional<bool> use_le;
   absl::optional<bool> use_all;
-  absl::optional<bool> use_analog_agc_agc2_level_estimator;
   absl::optional<bool> analog_agc_disable_digital_adaptive;
   absl::optional<int> agc_mode;
   absl::optional<int> agc_target_level;