Remove unused dev-dependency

Author: wooorm

package.json

@@ -31,7 +31,6 @@
   },
   "devDependencies": {
     "browserify": "^16.0.0",
-    "esmangle": "^1.0.0",
     "hastscript": "^4.0.0",
     "nyc": "^12.0.0",
     "prettier": "^1.13.5",