midi streamview: fix crash when switching playlists

We need to delete all existing region views before we redisplay the track. This
was removed as part of an "experimental performance optimization" in 4f7a4cd233
but playlist switching is a rare and non-performance limited operation

gtk2_ardour/streamview.cc

@@ -321,7 +321,7 @@ StreamView::playlist_switched (boost::weak_ptr<Track> wtr)
 	/* disconnect from old playlist */
 
 	playlist_connections.drop_connections ();
-	//undisplay_track ();
+	undisplay_track ();
 
 	/* draw it */
 	tr->playlist()->freeze();