bio.h: remove a very outdated comment

Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@fb.com>

include/linux/bio.h

 /*
- * 2.5 block I/O model
- *
  * Copyright (C) 2001 Jens Axboe <axboe@suse.de>
  *
  * This program is free software; you can redistribute it and/or modify