Remove links to Jetty API and documentation as they don't provide a versioned copy

[snicoll]

We are linking our Javadoc API to Jetty concepts but this breaks us every time the Jetty team releases a new version, see jetty/jetty.project#4546.

Let's remove the link until this issue is fixed and the Jetty team deploys Javadoc APIs we can link to reliably.

[wilkinsona]

This affects the reference documentation as well. It has a jetty-docs attribute for linking to Jetty's documentation