btrfs: scrub: remove redundant division of stripe_nr

Variable stripe_nr is being divided by map->num_stripes however the
result is never read. The division and assignment are redundant and
can be removed. Cleans up clang scan build warning:

fs/btrfs/scrub.c:1264:3: warning: Value stored to 'stripe_nr' is
never read [deadcode.DeadStores]

The code is a leftover from 6ded22c1 ("btrfs: reduce div64 calls by
limiting the number of stripes of a chunk to u32") that converted div64
to normal division, it's the same but previous version did not trigger a
warning.
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -1262,7 +1262,6 @@ static int get_raid56_logic_offset(u64 physical, int num,
 
 		/* Work out the disk rotation on this stripe-set */
 		rot = stripe_nr % map->num_stripes;
-		stripe_nr /= map->num_stripes;
 		/* calculate which stripe this data locates */
 		rot += i;
 		stripe_index = rot % map->num_stripes;