Remove NoCAmigration warning - should now be treated as an error (ATR-28537)

With !67267 (merged) we no longer have any unmigrated sequences. So we can remove this warning - unmigrated sequences (if they somehow reappear) will now crash the job.

Also added a warning message for the empty step workaround, although I don't think this is happening anywhere. Possibly it can be removed as well.