Remove unused npm dependency

Author: jackycute

package.json

@@ -21,7 +21,6 @@
     "express-session": "^1.14.0",
     "formidable": "^1.0.17",
     "helmet": "^2.1.2",
-    "highlight.js": "^9.5.0",
     "i18n": "^0.8.3",
     "imgur": "git+https://github.com/hackmdio/node-imgur.git",
     "jsdom-nogyp": "^0.8.3",