Revert "build: Add phan stub for ReturnTypeWillChange"

No longer needed since 5ceaf8a5d4, which added the
symfony/polyfill-php81 requirement to composer.json.

This reverts commit 7534655922.

Change-Id: I860f11180fe26c4626c467bdd298013236d70ad1

.phan/config.php

@@ -28,7 +28,6 @@ $cfg['minimum_target_php_version'] = '7.4.3';
 $cfg['file_list'] = array_merge(
 	$cfg['file_list'],
 	class_exists( Socket::class ) ? [] : [ '.phan/stubs/Socket.php' ],
-	class_exists( ReturnTypeWillChange::class ) ? [] : [ '.phan/stubs/ReturnTypeWillChange.php' ],
 	class_exists( AllowDynamicProperties::class ) ? [] : [ '.phan/stubs/AllowDynamicProperties.php' ],
 	class_exists( WeakMap::class ) ? [] : [ '.phan/stubs/WeakMap.php' ],
 	[

.phan/stubs/ReturnTypeWillChange.php

@@ -1,6 +0,0 @@
-<?php
-
-// Stub for PHP 8.1's ReturnTypeWillChange class
-#[Attribute]
-class ReturnTypeWillChange {
-}