block: remove the osdblk driver

This was just a proof of concept user for the SCSI OSD library, and
never had any real users.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Boaz Harrosh <ooo@electrozaur.com>
Signed-off-by: Jens Axboe <axboe@fb.com>