Now that oldref and l10n attributes were removed from
helpcontent in commit eb9ec1c794a0d3b8522375c7a87ac3ee999c8a66
we can as well drop support of these from l10ntools and xmlhelp

Change-Id: Ib6fe30e93f0d7e976bb675beee9395ab543af99c
Reviewed-on: https://gerrit.libreoffice.org/38715Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Andras Timar <andras.timar@collabora.com>