Change: Remove draw-io as default app

BREAKING CHANGE for administrators: draw-io has been removed as default web app. The app is now available via the [web-extensions repository](https://github.com/owncloud/web-extensions).

Please refer to [the web-app docs](https://owncloud.dev/services/web/#web-apps) to see how to load an external web app.

https://github.com/owncloud/web/pull/11252
https://github.com/owncloud/web/issues/11248
