Remove node_page_title() and entity_page_label() in favor of EntityInterface::label()

As of #2177041: Remove all implementations of hook_menu core no longer invokes the function node_page_title() and thus the function was removed.

The removal of hook_menu from where this function was previously used (originally invoked from node_menu()) is documented in https://www.drupal.org/node/1800686.

In Drupal 8.x you should now use the method EntityInterface::label() if you need the node title.

\Drupal\Core\Entity\Controller\EntityController::title can be used as a route title callback

Previously:

node_page_title($node);

Now:

$node->label();