btrfs: zoned: remove unused variable in btrfs_sb_log_location_bdev

This fixes warning:

fs/btrfs/zoned.c:491:6: warning: variable ‘zone_size’ set but not used [-Wunused-but-set-variable]
  491 |  u64 zone_size;

which got introduced in 12659251 ("btrfs: implement log-structured
superblock for ZONED mode"). We'll enable the warning by default and
want clean build until the relevant zoned patches land.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>