Track.h: removed unused class declaration "bbTrack"

The class forward declaration for "bbTrack" is neither used nor
required, therefore remove it.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
(cherry picked from commit 913aa5a6e8)

include/track.h

@@ -41,7 +41,6 @@
 class QMenu;
 class QPushButton;
 
-class bbTrack;
 class pixmapButton;
 class textFloat;
 class track;