Removal of apps/scenebuilder from OpenJFX repo

[Kevin Rushforth]

I filed the following JBS isuse to remova the SceneBuilder sources from 
the OpenJFX repo.

https://bugs.openjdk.java.net/browse/JDK-8198961

As mentioned in the Description of that issue, the OpenJFX repo contains 
the source code for a no-longer-maintained version of the SceneBuilder 
tool. Active development on SceneBuilder in the OpenJFX repo was stopped 
over three years ago. Since that time, the only changes have been either 
jigsaw-related changes to keep it buildable and runnable, or global 
changes that happened to touch some of the files in apps/scenebuilder.

A fork of SceneBuilder is maintained by Gluon, so anyone wanting to get 
the latest SceneBuilder should go there.

Before I proceed, I wanted to poll the list to see whether anyone has a 
concern with this. I don't plan to take any action for at least a week.

-- Kevin