It's not useful, OTOH it can happen that something goes wrong and the
result does not match the original.

One situation when this can happen is when non-XAdES signatures are
read, but the checkbox to write XAdES signature is enabled.

Change-Id: Icafad914175b29f7c0245220258bd1420ccd7b9c
Reviewed-on: https://gerrit.libreoffice.org/31437Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>