gh-75905: Remove test_xmlrpc_net: skipped since 2017 (#105796)

test_xmlrpc_net was skipped since 2017:
commit 73ffd3f2036179ed54591ef0455e5ba5694ae5bd.

The public buildbot.python.org server has no XML-RPC interface
anymore, and no replacement server was found in 6 years.