MDL-75952 general: Since php81, refection->setAccessible() is no-op

Refereces:
- https://wiki.php.net/rfc/make-reflection-setaccessible-no-op
- https://www.php.net/manual/en/reflectionproperty.setaccessible.php
- https://www.php.net/manual/en/reflectionmethod.setaccessible.php

As of PHP 8.1.0, calling this method has no effect; all methods are
invokable by default. So, let's remove all uses from core, they are
no-op.

lib/phpunit/classes/util.php

@@ -973,7 +973,6 @@ class phpunit_util extends testing_util {
     public static function call_internal_method($object, $methodname, array $params, $classname) {
         $reflection = new \ReflectionClass($classname);
         $method = $reflection->getMethod($methodname);
-        $method->setAccessible(true);
         return $method->invokeArgs($object, $params);
     }