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}

webrtc/modules/audio_coding/neteq/neteq_impl.h

@@ -123,6 +123,8 @@ class NetEqImpl : public webrtc::NetEq {
   // -1 on failure.
   int RemovePayloadType(uint8_t rtp_payload_type) override;
 
+  void RemoveAllPayloadTypes() override;
+
   bool SetMinimumDelay(int delay_ms) override;
 
   bool SetMaximumDelay(int delay_ms) override;