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/external/tests/external_api_test.php

@@ -469,7 +469,6 @@ class external_api_test extends \advanced_testcase {
     protected function get_context_from_params() {
         $rc = new \ReflectionClass(external_api::class);
         $method = $rc->getMethod('get_context_from_params');
-        $method->setAccessible(true);
         return $method->invokeArgs(null, func_get_args());
     }
 }