Since it's not used outside xmloff.

Change-Id: Ibec67db2106300b6d4a91ce07d779e16e028eb2a
Reviewed-on: https://gerrit.libreoffice.org/38144Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>