and document in the registry that the SubstFontsPS is unused

Change-Id: I66a9e0b4353f17b34cffb02823726c9887b7bdd3
Reviewed-on: https://gerrit.libreoffice.org/64065
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>