Pass by reference in notifications_watchdog_notifications($op, &$arg0) breaks in PHP 5.3

The argument $arg0 is not necessary anyway, best to just remove it.

More details on the issue:
- The hook_notifications is, eventually, invoked from messaging.module in messaging_module_invoke_all()
- messaging_module_invoke_all() uses call_user_func_array() to make the invokation
- call_user_func_array() does not work for functions which request pass by reference (it returns NULL).