FindXMLRPC: Tolerate no include directories
xmlrpc-c-config $modules --cflags may report no -I flags if the
headers are in a standard location like /usr/include. In this case
it is okay for XMLRPC_INCLUDE_DIRS to be empty.
Ideally the --cflags output should be used as hints for a find_path
call to really find the headers as we do for other find modules, but
simply assuming no explicit include directories are needed is good
enough for now.
While at it, remove some unused code paths.
Fixes: #17347