Use case: the bugdoc has 2 signatures, one normal one and one with
SubFilter=ETSI.RFC3161. By not hiding the second signature it's possible
to counter-sign the document, even if we don't handle the contents of
the second one.

Change-Id: I580e1211072ec9839f01b529b569c98b702b6534
Reviewed-on: https://gerrit.libreoffice.org/31539Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>