Block Editor: Declare the wp_template post type as built-in.

Remove redundant `show_in_admin_bar` property, which defaults to the value of `show_in_menu`.

Follow-up to [51003].

See #53176.

git-svn-id: https://develop.svn.wordpress.org/trunk@51014 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -336,10 +336,10 @@ function create_initial_post_types() {
 			),
 			'description'           => __( 'Templates to include in your theme.' ),
 			'public'                => false,
+			'_builtin'              => true, /* internal use only. don't use this when registering your own post type. */
 			'has_archive'           => false,
 			'show_ui'               => false,
 			'show_in_menu'          => false,
-			'show_in_admin_bar'     => false,
 			'show_in_rest'          => true,
 			'rewrite'               => false,
 			'rest_base'             => 'templates',