InstrumentTrack: presetPreviewPlayHandle no longer a friend class

For a long time presetPreviewPlayHandle has been a friend class of
InstrumentTrack as it had to modify MidiPort settings. However the
InstrumentTrack's MidiPort is now accessible via getMidiPort() so
there's no need for keeping presetPreviewPlayHandle a friend class
anymore.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>

include/instrument_track.h

@@ -53,7 +53,6 @@ class midiPortMenu;
 class multimediaProject;
 class notePlayHandle;
 class pluginView;
-class presetPreviewPlayHandle;
 class tabWidget;
 class trackLabelButton;
 
@@ -233,7 +232,6 @@ private:
 	friend class instrumentTrackView;
 	friend class instrumentTrackWindow;
 	friend class notePlayHandle;
-	friend class presetPreviewPlayHandle;
 	friend class flpImport;
 
 } ;