Code Modernization: Remove unused $undefined property in Tests_WP_Customize_Manager.

This appears to be a copy/paste from the `Tests_WP_Customize_Setting` class, and is not actually used in `WP_Customize_Manager` tests.

Follow-up to [31329], [32658], [53851].

Props jrf.
See #56033.

git-svn-id: https://develop.svn.wordpress.org/trunk@53855 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/customize/manager.php

@@ -19,13 +19,6 @@ class Tests_WP_Customize_Manager extends WP_UnitTestCase {
 	 */
 	public $manager;
 
-	/**
-	 * Symbol.
-	 *
-	 * @var stdClass
-	 */
-	public $undefined;
-
 	/**
 	 * Admin user ID.
 	 *