I've noticed the new FSDT Universal Installer does not offer a way to completely uninstall already existing P3D scenery.
That's normal, of course.
Since it doesn't yet have a way to INSTALL an FSX/P3D scenery as well, it would be wrong if it let you Uninstall one, because it would have left the Uninstaller ( which was created by the normal *installer* ) "orphaned", breaking the sync between install and uninstall.
Like any other Windows program, they are uninstalled from the Windows Control Panel.