Remove unused and unneeded MIDI methods and variables

The fields in AudioIO are simply unneeded (commented out, declared but never implemented, or otherwise unused).

mLastMidiPosition (in NoteTrack) looks like the vestiges of a far older attempt at MIDI playback; the field itself is completely unused though so it is safe to remove it.

src/AudioIO.h

@@ -440,11 +440,8 @@ private:
    void OutputEvent();
    void FillMidiBuffers();
    void GetNextEvent();
-   void AudacityMidiCallback();
    double AudioTime() { return mT0 + mNumFrames / mRate; }
    double PauseTime();
-   // double getCurrentTrackTime();
-   // long CalculateMidiTimeStamp(double time);
    void AllNotesOff();
 #endif
 
@@ -565,7 +562,6 @@ private:
    bool             mMidiOutputComplete;
    /// Is the next event a note-on?
    bool             mNextIsNoteOn;
-   //   int                 mCnt;
    /// mMidiStreamActive tells when mMidiStream is open for output
    bool             mMidiStreamActive;
    /// when true, mSendMidiState means send only updates, not note-on's,