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.

communication/provider/matrix/tests/local/command_test.php

@@ -281,7 +281,6 @@ class command_test extends \advanced_testcase {
         );
 
         $execute = new ReflectionMethod($instance, 'execute');
-        $execute->setAccessible(true);
         $execute->invoke($instance, $command);
     }
 

communication/provider/matrix/tests/matrix_client_test.php

@@ -447,7 +447,6 @@ class matrix_client_test extends \advanced_testcase {
 
         $rc = new \ReflectionClass($instance);
         $rcm = $rc->getMethod('execute');
-        $rcm->setAccessible(true);
         $result = $rcm->invoke($instance, $command);
 
         $this->assertEquals(200, $result->getStatusCode());