AcmReceiver: Ask NetEq to delete all decoders at once instead of one by one

It requires a new NetEq method, but it can no longer fail. And we no
longer need to use AcmReceiver::decoders_, which we're trying to
eliminate.

(This is a re-land of https://codereview.webrtc.org/2342313002.)

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2348233002
Cr-Commit-Position: refs/heads/master@{#14304}
diff --git a/webrtc/modules/audio_coding/acm2/acm_receiver.h b/webrtc/modules/audio_coding/acm2/acm_receiver.h
index d39581e..53def2b 100644
--- a/webrtc/modules/audio_coding/acm2/acm_receiver.h
+++ b/webrtc/modules/audio_coding/acm2/acm_receiver.h
@@ -186,7 +186,7 @@
   //
   // Remove all registered codecs.
   //
-  int RemoveAllCodecs();
+  void RemoveAllCodecs();
 
   // Returns the RTP timestamp for the last sample delivered by GetAudio().
   // The return value will be empty if no valid timestamp is available.