zfs_btree_remove_idx
zfs_btree_remove_idx(bt, &bt_idx);
zfs_btree_remove_idx(tree, &where);
zfs_btree_remove_idx(&rt->rt_root, &where_before);
zfs_btree_remove_idx(&rt->rt_root, &where);
void zfs_btree_remove_idx(zfs_btree_t *, zfs_btree_index_t *);