We don't need to create an URL when we get the XGraphic from the
signatures, that use SimpleShape. We can just use the XGraphic
directly and set the "Graphic" property on creation of
SimpleShape.

Change-Id: I82b1f76cd358fca29f5b9dfbed0e735bd1f38e15
Reviewed-on: https://gerrit.libreoffice.org/50141Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>