This is unused since commit 2303d4f1
([API CHANGE] offapi: remove unused css.xml.crypto.XMLEncryption,
2017-07-18).

Change-Id: Icaf5054f13271a9040137f305bd7be40d1be6600
Reviewed-on: https://gerrit.libreoffice.org/40699Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>