Determines if a node can be removed from the book.
A node can be removed from a book if it is actually in a book and it either is not a top-level page or is a top-level page with no children.
$node: The node to remove from the outline.
function _book_node_is_removable($node) {
return !empty($node->book['bid']) && ($node->book['bid'] != $node->nid || !$node->book['has_children']);
}