REMOVE typecasting of WP_Admin_Bar in TablePress_Admin_Controller::add_wp_admin_bar_new_content_menu_entry()

It is possible to completely override the WP_Admin_Bar class using the 'wp_admin_bar_class' filter in wp-includes/admin-bar.php

So, typecasting this method's argument as such can cause a Fatal error should that class not be instantiated.

This pull request prevents that from happening.