#14 Remove the signxml-openadr dependency after the merge request is approved

I released a temporary version of SignXML under the name signxml-openadr with support for the ReplayProtect elements in the Signature of a message. I offered a pull request to the maintainer of signxml for inclusion. After that pull request is accepted, we can then again depend on signxml directly.

The merge request is now approved, I will wait for a new release of signxml before removing our own temporary dependency.