Remove old version of SceneBuilder from OpenJFX

The apps/scenebuilder directory contains the source code for a no-longer-maintained version of the SceneBuilder tool. I propose to remove the entire apps/scenebuilder directory tree.

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.