MDL-70877 php: remove final functions __clone and __wakeup

Implementing these functions as final does not prevent the object to be
cloned or serialized, so the comment to them was wrong.
However having these functions not public breaks in PHP 8.0.
It still does not make sense to clone or serialise file_system or external_settings classes
but there are a lot of classes in moodle that should not be cloned, no reason to have
a specific solution here.

lib/externallib.php

@@ -1232,12 +1232,6 @@ protected function __construct() {
         }
     }
 
-    /**
-     * Clone - private - can not be cloned
-     */
-    private final function __clone() {
-    }
-
     /**
      * Return only one instance
      *

lib/filestorage/file_system.php

@@ -34,20 +34,6 @@
  */
 abstract class file_system {
 
-    /**
-     * Private clone method to prevent cloning of the instance.
-     */
-    final protected function __clone() {
-        return;
-    }
-
-    /**
-     * Private wakeup method to prevent unserialising of the instance.
-     */
-    final protected function __wakeup() {
-        return;
-    }
-
     /**
      * Output the content of the specified stored file.
      *