remove no-op for sitemap manipulator
remove no-op for sitemap manipulator

not necessary now that sitemap is not longer a part of preloading for config, see: https://github.com/middleman/middleman/commit/5337d0640bdd35ad2c22d02ad58319a1ca230c84
middleman/middleman-sprockets,middleman/middleman-sprockets,middleman/middleman-sprockets