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.

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2342313002
Cr-Commit-Position: refs/heads/master@{#14275}

webrtc/modules/audio_coding/neteq/include/neteq.h

@@ -183,6 +183,9 @@ class NetEq {
   // -1 on failure.
   virtual int RemovePayloadType(uint8_t rtp_payload_type) = 0;
 
+  // Removes all payload types from the codec database.
+  virtual void RemoveAllPayloadTypes() = 0;
+
   // Sets a minimum delay in millisecond for packet buffer. The minimum is
   // maintained unless a higher latency is dictated by channel condition.
   // Returns true if the minimum is successfully applied, otherwise false is