remove broccoli-clean-css dependency

no issue
- we're now using postcss with the cssnano plugin when building the production files so we don't need the additional minification plugin
- fixes `ember build -prod`

package.json

@@ -30,7 +30,6 @@
     "blueimp-md5": "2.7.0",
     "bower": "1.8.0",
     "broccoli-asset-rev": "2.5.0",
-    "broccoli-clean-css": "2.0.1",
     "broccoli-concat": "3.2.2",
     "broccoli-funnel": "1.1.0",
     "broccoli-merge-trees": "1.2.1",