use official cli way of adding playground to dev deps on ng add

[timstoddard]

Desired Behavior

Use the CLI's new way of defining how a package installed with "ng add" should be stored in the package.json.
Original bug report
PR with fix

Since we want to save Playground in the devDependencies, we can now simply add to our package.json:

"ng-add": {
  "save": 'devDependencies'
}

Also we can get rid of our boilerplate that is currently handling this.