deleted unused file
for simple artifacts like this it's convenient to keep everything in one file
no reason for this to not be thread-safe
cleanup
test fixes - Grails 2.3 has Spock support built in, so using the plugin isn't needed and causes the tests to fail. Note that the 'Should fail early if no server url has been configured' test in ConfigSitemapServerUrlResolverSpec fails, but it's a bad test - if there's nothing in the config it returns a non-null default url