block: remove REQ_HARDBARRIER

REQ_HARDBARRIER is dead now, so remove the leftovers.  What's left
at this point is:

 - various checks inside the block layer.
 - sanity checks in bio based drivers.
 - now unused bio_empty_barrier helper.
 - Xen blockfront use of BLKIF_OP_WRITE_BARRIER - it's dead for a while,
   but Xen really needs to sort out it's barrier situaton.
 - setting of ordered tags in uas - dead code copied from old scsi
   drivers.
 - scsi different retry for barriers - it's dead and should have been
   removed when flushes were converted to FS requests.
 - blktrace handling of barriers - removed.  Someone who knows blktrace
   better should add support for REQ_FLUSH and REQ_FUA, though.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/usb/storage/uas.c

@@ -331,10 +331,7 @@ static struct urb *uas_alloc_cmd_urb(struct uas_dev_info *devinfo, gfp_t gfp,
 
 	iu->iu_id = IU_ID_COMMAND;
 	iu->tag = cpu_to_be16(stream_id);
-	if (sdev->ordered_tags && (cmnd->request->cmd_flags & REQ_HARDBARRIER))
-		iu->prio_attr = UAS_ORDERED_TAG;
-	else
-		iu->prio_attr = UAS_SIMPLE_TAG;
+	iu->prio_attr = UAS_SIMPLE_TAG;
 	iu->len = len;
 	int_to_scsilun(sdev->lun, &iu->lun);
 	memcpy(iu->cdb, cmnd->cmnd, cmnd->cmd_len);