Time track properly warps the MIDI event times (but not pitches)

mMidiPlaySpeed doesn't handle all of the options provided by time tracks,
and in fact doesn't even use time tracks.  Using the time track
instead allows note tracks to remain synchronized with the rest of the audio.

mMidiPlaySpeed _did_ work where it was used in the transcription tool
bar, but that already updates a time track so no special handling is
needed.

src/AudioIO.h

@@ -56,7 +56,6 @@ class TimeTrack;
 class AudioThread;
 class Meter;
 class SelectedRegion;
-class TimeTrack;
 
 class AudacityProject;
 
@@ -227,9 +226,6 @@ class AUDACITY_DLL_API AudioIO final {
  public:
    bool SetHasSolo(bool hasSolo);
    bool GetHasSolo() { return mHasSolo; }
-   /// Sets the speed for midi playback, based off of the transcription speed.
-   /// This takes a percentage, so passing 100 will play at normal speed.
-   void SetMidiPlaySpeed(double s) { mMidiPlaySpeed = s * 0.01; }
 #endif
 
    /** \brief Returns true if the stream is active, or even if audio I/O is
@@ -527,9 +523,6 @@ private:
    long             mMidiLatency;
    /// Latency of MIDI synthesizer
    long             mSynthLatency;
-   /// A copy of TranscriptionToolBar::mPlaySpeed - a linear speed offset.
-   /// Should be replaced with use of mTimeTrack
-   double           mMidiPlaySpeed;
 
    // These fields are used to synchronize MIDI with audio: