Revert "Polymer elements added to third_party/polymer."

third_party/polymer/platform/bower.json

 {
   "name": "platform",
   "main": "platform.js",
   "homepage": "https://github.com/Polymer/platform",
   "authors": [
     "The Polymer Authors"
   ],
   "description": "Integrate platform polyfills: load, build, test",
   "keywords": [
     "polymer",
     "web",
     "components"
   ],
   "license": "BSD",
   "private": true
 }