btrfs: scrub: remove unused nocow worker pointer

The member btrfs_fs_info::scrub_nocow_workers is unused since the nocow
optimization was removed from scrub in 9bebe665

 ("btrfs: scrub:
Remove unused copy_nocow_pages and its callchain").

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>