REST API: Ensure rest_controller instantiates the post type's declared REST controller class.

Ensures that the ::get_rest_controller() method will always return an instanceof the expected controller class, or null.
Removes unused private static property $post_type_controllers.

Props dlh, TimothyBlynJacobs.
Fixes #45677.



git-svn-id: https://develop.svn.wordpress.org/trunk@46435 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -15,15 +15,6 @@
  * @see WP_REST_Controller
  */
 class WP_REST_Posts_Controller extends WP_REST_Controller {
-
-	/**
-	 * Instances of post type controllers keyed by post type.
-	 *
-	 * @since 5.3.0
-	 * @var WP_REST_Controller[]
-	 */
-	private static $post_type_controllers = array();
-
 	/**
 	 * Post type.
 	 *