Add an unbind command to services config

The idea is to remove the mapping from default-services.yaml so that trying to require this service will get an error.

The reason this is needed is that the binder needs to register all bound classes to get ones depended on by required.

The reason that's sometimes a problem is that the class might not load in some environments. For instance, in a servlet container, it's illegal for a .jar to redefine javax.servlet.*. So the dependency that has that is rejected. Really, it's silly to try to have Jetty present at all in such a situation, but that's a little more significant refactoring.