Remove npm reference to unused zeroclipboard.

web/react/package.json

@@ -10,7 +10,6 @@
     "keymirror": "0.1.1",
     "marked": "0.3.5",
     "object-assign": "3.0.0",
-    "react-zeroclipboard-mixin": "0.1.0",
     "twemoji": "1.4.1"
   },
   "devDependencies": {