No more disk-reader roll-delay

It was not working in sdio/6.0-pre anyway and with upcoming changes to
latency compensation the concept of per disk[stream/reader] will go away.

scripts/_dump_latency.lua

@@ -8,7 +8,6 @@ function factory () return function ()
 	print ("Worst Output Latency:  ", Session:worst_output_latency ())
 	print ("Worst Input Latency:   ", Session:worst_input_latency ())
 	print ("Worst Track Latency:   ", Session:worst_track_latency ())
-	print ("Worst Track Roll Delay:", Session:worst_track_roll_delay ())
 	print ("Worst Playback Latency:", Session:worst_playback_latency ())
 	print (" -- Tracks --")
 	for t in Session:get_tracks ():iter () do