Delete VAD methods from AcmReceiver and move functionality inside NetEq

This change essentially does two things:

1. Remove the VAD-related methods from AcmReceiver. These are
EnableVad(), DisableVad(), and vad_enabled(). None of them were used
outside of unit tests.

2. Move the functionality to set AudioFrame::speech_type_ and
AudioFrame::vad_activity_ inside NetEq. This was previously done in
AcmReceiver, but based on information inherently owned by NetEq.

With the change in 2, NetEq's GetAudio interface can be simplified by
removing the output type parameter. This will be done in a follow-up
CL.

BUG=webrtc:5607

Review URL: https://codereview.webrtc.org/1772583002

Cr-Commit-Position: refs/heads/master@{#11902}

webrtc/modules/audio_coding/neteq/neteq_impl.h

@@ -379,6 +379,8 @@ class NetEqImpl : public webrtc::NetEq {
   bool enable_fast_accelerate_ GUARDED_BY(crit_sect_);
   std::unique_ptr<Nack> nack_ GUARDED_BY(crit_sect_);
   bool nack_enabled_ GUARDED_BY(crit_sect_);
+  AudioFrame::VADActivity last_vad_activity_ GUARDED_BY(crit_sect_) =
+      AudioFrame::kVadPassive;
 
  private:
   RTC_DISALLOW_COPY_AND_ASSIGN(NetEqImpl);