Remove region-output export (GUI)

This use-case is better served via stem-export.

This export mode never worked correctly when latent plugins
are present on the track. or when realtime export was chosen.

This used to call
  track.export_stuff() -> bounce_process()
periodically in small chunks from freewheeling callback,
which is no longer functional.

libs/ardour/ardour/export_channel.h

@@ -105,7 +105,6 @@ class LIBARDOUR_API RegionExportChannelFactory
 		None,
 		Raw,
 		Fades,
-		Processed
 	};
 
 	RegionExportChannelFactory (Session * session, AudioRegion const & region, AudioTrack & track, Type type);