AFAIK, you can't disable session cookies in Jetty by means of declaration, you need to do it programmatically. According to this post, the following code does the trick:
((HashSessionManager)context.getSessionHandler().getSessionManager())
.setUsingCookies(false);
The RAP project used to provide a fragment bundle for jetty that contains a customizer class which disables sessions using this method. This customizer has to be enabled with the system property you referred to, but this only works when the fragment is present in your environment.
This “jettycustomizer” bundle has been dropped when RAP enabled support for multi-tab browsing, but you can still get it from the 2.1 branch.