btrfs: scrub: remove unused scrub_ctx::sectors_per_bio

The recent scrub rewrite forgot to remove the sectors_per_bio in
6.3 in 13a62fd9 ("btrfs: scrub: remove scrub_bio structure").
This was found by tool https://github.com/jirislaby/clang-struct .
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -192,7 +192,6 @@ struct scrub_ctx {
 	int			cur_stripe;
 	atomic_t		cancel_req;
 	int			readonly;
-	int			sectors_per_bio;
 
 	/* State of IO submission throttling affecting the associated device */
 	ktime_t			throttle_deadline;