The signature manager always creates an XML helper, and optionally creates a
PDF helper as well. Both of them initialize xmlsec, and when the signature
manager is deleted, there are two de-inits, leading to an assertion failure in
xmlsec.

Fix the problem by moving the duplicated xmlsec init to the signature manager.
This has the additional benefit that general security-related code no longer
has to talk to the XML helper, it can use the signature manager, which feels
more natural. (What viewing a certificate had to do with XML?)

Change-Id: If6a6bc433636445f3782849a367d4a7ac0be7688
Reviewed-on: https://gerrit.libreoffice.org/30695Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>