unnecessary query on book's nodeapi load

In nodeapi load a query is done whether it is a book node or not. This was reported for d7 and d6 too (http://drupal.org/node/251239) and committed to head. For 5 it needs a different approach though because since 6 all node types can be a book page.

Corrected to:

function book_nodeapi(&$node, $op, $teaser, $page) {
  switch ($op) {
    case 'load':
      if ($node->type == 'book') {
        return db_fetch_array(db_query('SELECT parent, weight FROM {book} WHERE vid = %d', $node->vid));
      }