(#8296) Remove XMLRPC code
The XMLRPC code was left in place to ensure backward compatibility with
Puppet 0.24.x clients, but 0.24.x clients haven't been able to talk to
masters since 0.25.6 due to other bugs and incompatibilities.
Therefore, the XMLRPC code has been dead code for a long time and
removing it makes the codebase a lot easier to navigate.
Paired with Patrick Carlisle <patrick@puppetlabs.com>