Exclude jackrabbit-webapp from our fork as it includes oak dependencies which heavily confuses IDEs

The jackrabbit-webapp includes a dependency on oak-jcr, which itself depends on (typically other versions of) jackrabbit dependencies (api, commons, etc.).
That dauses IDEs like IntellliJ to get really confused when trying to lookup an implementation class, often resolving to the (not loaded) version used by oak

As the jackrabbit-webapp isn't in any way needed/used for our product or client implementations, removing it from (the build of) our fork therefore is a trivial fix.