package/xml-security-c: remove package

https://santuario.apache.org/
"July 2024
 As announced early this year, the C++ library has been officially
 retired. A fork of this code base has been migrated to the Shibboleth
 Project, which has been the sole maintainer for a number of years now.
 See the Shibboleth wiki for notable caveats regarding usage of this
 code."

https://shibboleth.atlassian.net/wiki/spaces/DEV/pages/3726671873/Santuario
"The C++ xml-security-c library has been retired at Apache and is now
 solely maintained by the Shibboleth Project for our use only. Use by
 any other project, while permitted freely under the license, is
 ill-advised and unsupported by the Shibboleth Project. While we
 continue to publish a fork of the code in our repository, and will
 accept bug reports and certainly security reports, we do not promise
 any responsiveness to third parties, and urge that no other projects
 rely on this code. Existing users should seek alternatives or consider
 forking the code for their own use."

The package is unmaintained and will cause build errors with the
upcoming bump of OpenSSL to 4.0.0. No other package depends on it so we
can safely remove it from buildroot.

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>

Config.in.legacy

@@ -146,6 +146,13 @@ endif
 
 comment "Legacy options removed in 2026.05"
 
+config BR2_PACKAGE_XML_SECURITY_C
+	bool "xml-security-c removed"
+	select BR2_LEGACY
+	help
+	  xml-security-c was no longer maintained upstream, so it has
+	  been dropped.
+
 config BR2_PACKAGE_LIBPHIDGET
 	bool "libphidget"
 	select BR2_LEGACY