fix clean-css dependency

Author: rickkas7

package-lock.json

@@ -18,7 +18,6 @@
         "apidoc": "^0.17.5",
         "autoprefixer": "^7.1.5",
         "binary-version-reader": "^2.0.1",
-        "clean-css": "^5.1.2",
         "del": "^3.0.0",
         "gulp": "^4.0.2",
         "gulp-footer": "^1.0.5",