btrfs: delete unused member nobarriers

The last consumer of nobarriers is removed by the commit [1] and sync
won't fail with EOPNOTSUPP anymore. Thus, now when write cache is write
through it just return success without actually transpiring such a
request to the block device/lun.

[1]
commit b25de9d6
block: remove BIO_EOPNOTSUPP

And, as the device/lun write cache state may change dynamically saving
such as state won't help either. So deleting the member nobarriers.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3521,9 +3521,6 @@ static int write_dev_flush(struct btrfs_device *device, int wait)
 	struct bio *bio;
 	int ret = 0;
 
-	if (device->nobarriers)
-		return 0;
-
 	if (wait) {
 		bio = device->flush_bio;
 		if (!bio)

fs/btrfs/volumes.h

@@ -123,7 +123,6 @@ struct btrfs_device {
 	struct list_head resized_list;
 
 	/* for sending down flush barriers */
-	int nobarriers;
 	struct bio *flush_bio;
 	struct completion flush_wait;