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

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.