Reland "Deprecate the adaptive level controller"

This is a reland of 6f37ed78d99daa36e964ff0a65b205f0916d9949

CQ dry run OK except for missing iOS swarming bots.
NOTRY=True

Original change's description:
> Deprecate the adaptive level controller
>
> Level control handled by default-on AGC.
>
> Bug: none
> Change-Id: I405daeceece12c896d41156b649fcfd556726f77
> Reviewed-on: https://webrtc-review.googlesource.com/59682
> Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
> Reviewed-by: Alex Loiko <aleloi@webrtc.org>
> Commit-Queue: Sam Zackrisson <saza@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22305}

Bug: none
Change-Id: I0b9b8e2f3457d5efd4603efbfbbc6b84651df315
Reviewed-on: https://webrtc-review.googlesource.com/60720
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22352}
diff --git a/modules/audio_processing/include/audio_processing.h b/modules/audio_processing/include/audio_processing.h
index 7057f28..33ecf89 100644
--- a/modules/audio_processing/include/audio_processing.h
+++ b/modules/audio_processing/include/audio_processing.h
@@ -211,8 +211,8 @@
 // AudioProcessing* apm = AudioProcessingBuilder().Create();
 //
 // AudioProcessing::Config config;
-// config.level_controller.enabled = true;
 // config.high_pass_filter.enabled = true;
+// config.gain_controller2.enabled = true;
 // apm->ApplyConfig(config)
 //
 // apm->echo_cancellation()->enable_drift_compensation(false);
@@ -262,14 +262,6 @@
   // by changing the default values in the AudioProcessing::Config struct.
   // The config is applied by passing the struct to the ApplyConfig method.
   struct Config {
-    struct LevelController {
-      bool enabled = false;
-
-      // Sets the initial peak level to use inside the level controller in order
-      // to compute the signal gain. The unit for the peak level is dBFS and
-      // the allowed range is [-100, 0].
-      float initial_peak_level_dbfs = -6.0206f;
-    } level_controller;
     struct ResidualEchoDetector {
       bool enabled = true;
     } residual_echo_detector;