The only remaining task it did was StartVerifySignatureElement() in
startElement(), but that can be done in OOXMLSecParser / XSecParser
easily, no need to have separate class for that.

Change-Id: I0863019d3a92a911df0d60bf872dd0978eac03d5
Reviewed-on: https://gerrit.libreoffice.org/36770Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>