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

Change-Id: Iefe201a9e93fc02e1295d23ad1b6c27286547241
Reviewed-on: https://gerrit.libreoffice.org/40434Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>