Bug 1145083 - Remove unused grunt-filerev node package

We switched to grunt-cache-busting some time ago and so no longer use
grunt-filerev.

package.json

@@ -14,7 +14,6 @@
     "grunt-contrib-copy": "~0.5.0",
     "grunt-contrib-cssmin": "^0.9.0",
     "grunt-contrib-uglify": "^0.4.0",
-    "grunt-filerev": "^0.2.1",
     "grunt-html-angular-validate": "^0.4.0",
     "grunt-usemin": "~2.1.1",
     "karma": "~0.12.0",