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

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

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

Comment From: wilkinsona

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