Remove remaining/stray menu_link references from book module code

$ grep -R menu_link core/modules/book/
core/modules/book/lib/Drupal/book/BookManager.php:        // After _menu_link_translate(), $item['title'] has the localized link title.
core/modules/book/lib/Drupal/book/BookManager.php:   *   fields from the {menu_links} table, and optionally additional information
core/modules/book/lib/Drupal/book/Form/BookAdminEditForm.php:use Drupal\menu_link\MenuLinkStorageControllerInterface;
core/modules/book/lib/Drupal/book/BookManagerInterface.php:   * Note: copied from _menu_link_translate() in menu.inc, but reduced to the
core/modules/book/book.module: * Implements hook_menu_link_defaults().
core/modules/book/book.module:function book_menu_link_defaults() {

- I think the first one should mention the bookLinkTranslate() method now?
- The second is part of the menu_tree_data method, and should probably be updated too, or was that left like that for a reason?
- The others are fine, the use is removed in #2209029: Follow-up: clean up use statements in book module code.

This is not a docs issue.

1. +++ b/core/modules/book/book.module
   @@ -715,6 +718,27 @@ function template_preprocess_book_node_export_html(&$variables) {
   +/**
   + * Returns HTML for a wrapper for a book sub-tree.
   + *
   + * @param $variables
   + *   An associative array containing:
   + *   - tree: An HTML string containing the tree's items.
   + *
   + * @see template_preprocess_menu_tree()
   + * @ingroup themeable
   + */
   +function theme_book_tree($variables) {
   +  return '<ul class="menu">' . $variables['tree'] . '</ul>';
   +}
   +
   

   Just wonder why we don't already create a twig template, which would open up another issue otherwise.

2. +++ b/core/modules/book/lib/Drupal/book/BookManager.php
   @@ -921,7 +921,7 @@ protected function _menu_tree_check_access(&$tree) {
   +        // After bookLinkTranslate(), $item['title'] has the translated title.
   

   we could use static::bookLinkTranslate

We could do more obviously, but let's stay with the these bits for now.

Committed to 8.x. Thanks.