Improve uc_cart_get_contents() query

Noted when fixing #1305634: Deduplicate uc_cart_get_item() and uc_cart_get_contents()

    $result = db_query("SELECT c.*, n.title, n.vid FROM {node} n INNER JOIN {uc_cart_products} c ON n.nid = c.nid WHERE c.cart_id = :id ORDER BY c.cart_item_id ASC", array(':id' => $cid));

    foreach ($result as $item) {
      $product = node_load($item->nid);

We don't need the join at all; we can just get the title and vid from the node_load() result.