refactor(grunt:ripple): remove superfluous cordova prepare

Ripple executes _cordova prepare_ internally on browser refresh (see [node_modules/ripple-emulator/lib/server/emulate/hosted.js](https://github.com/apache/incubator-ripple/blob/master/lib/server/emulate/hosted.js)). Thus, it's not necessary to call the Grunt prepare task to move files from www/ to the respective platform www directories.