Upgrade/Install: Revert [59981] pending test failure investigation.

See #62718.

git-svn-id: https://develop.svn.wordpress.org/trunk@59982 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-upgrader.php

@@ -1007,17 +1007,9 @@ public function run( $options ) {
 	public function maintenance_mode( $enable = false ) {
 		global $wp_filesystem;
 
-		if ( ! function_exists( 'WP_Filesystem' ) ) {
-			require_once ABSPATH . '/wp-admin/includes/file.php';
-		}
-
-		ob_start();
-		$credentials = request_filesystem_credentials( '' );
-		ob_end_clean();
-
-		if ( false === $credentials || ! WP_Filesystem( $credentials ) ) {
-			wp_trigger_error( __FUNCTION__, __( 'Could not access filesystem.' ) );
-			return;
+		if ( ! $wp_filesystem ) {
+			require_once ABSPATH . 'wp-admin/includes/file.php';
+			WP_Filesystem();
 		}
 
 		$file = $wp_filesystem->abspath() . '.maintenance';