Upload: Remove redundant check for ABSPATH in wp-admin/async-upload.php.

This aims to bring consistency with a similar fragment in other files, since relocating `wp-admin` or `wp-load.php` is not supported at this time.

Follow-up to [6659], [7971], [8315].

Props hussain896, swissspidy, knutsp, SergeyBiryukov.
Fixes #62809.
Built from https://develop.svn.wordpress.org/trunk@59634


git-svn-id: http://core.svn.wordpress.org/trunk@58997 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/admin.php

@@ -31,6 +31,7 @@ if ( isset( $_GET['import'] ) && ! defined( 'WP_LOAD_IMPORTERS' ) ) {
 	define( 'WP_LOAD_IMPORTERS', true );
 }
 
+/** Load WordPress Bootstrap */
 require_once dirname( __DIR__ ) . '/wp-load.php';
 
 nocache_headers();