block: kill QUEUE_FLAG_FLUSH_NQ

We have various helpers for setting/clearing this flag, and also
a helper to check if the queue supports queueable flushes or not.
But nobody uses them anymore, kill it with fire.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/ata/libata-scsi.c

@@ -1318,8 +1318,6 @@ static int ata_scsi_dev_config(struct scsi_device *sdev,
 		scsi_change_queue_depth(sdev, depth);
 	}
 
-	blk_queue_flush_queueable(q, false);
-
 	if (dev->flags & ATA_DFLAG_TRUSTED)
 		sdev->security_supported = 1;