Remove the dead code inside the FastTree assembly

[sfilipi]

The code inside src/Microsoft.ML.FastTree/Application is not in use.

Delete the Folder.

[codemzs]

Have you confirmed this is not used by internal repo? If it is we should make sure we move these files there after they are removed from here.

[Ivanidzo4ka]

They are used, but I don't see huge issues in restoring this files internally.
I'm closing issue, since files and catalog no longer part of this repository.