scoutfs_item_set_batch() has a rocky history of being a giant pain in
the butt. It's been a lot simpler to have callers use individual item
ops instead of trying to describe a compound item operation to sometihng
like _set_batch().
Its last user has gone away so we can remove it and never speak of it
again. And there was much rejoycing.
Signed-off-by: Zach Brown <zab@versity.com>