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.

question/engine/tests/question_engine_test.php

@@ -299,7 +299,6 @@ class question_engine_test extends advanced_testcase {
         $renderer = new \core_question_renderer($PAGE, 'core_question');
         $rc = new \ReflectionClass($renderer);
         $rcm = $rc->getMethod('number');
-        $rcm->setAccessible(true);
 
         $this->assertEquals($expected, $rcm->invoke($renderer, $value));
     }