security/xml-security-c: remove as no longer needed, retired upstream

Imported a few years back as dep for security/libdigidocpp which as of today no longer
uses it; no other port picked it up. WWW shows the last release in 2021 and upstream
officially abandoning their C++ library.

OK to remove?

---
Information for inst:xml-security-c-2.0.4p3

Comment:
C++ implementation of W3C security standards for XML

Description:
Apache XML Security for C++: This library includes a mature Digital Signature
and Encryption implementation using a proprietary C++ API on top of the Xerces-C
XML Parser's DOM API. It includes a pluggable cryptographic layer, but support
for alternatives to OpenSSL are less complete and less mature.

Maintainer: Klemens Nanni <kn@openbsd.org>

WWW: https://santuario.apache.org