Remove `config.js` in all apps

[gciotola]

What I did

From version 3.0.0 of app-elements we don't need anymore to configure the app domain via config.js and other variants. So I have removed all reference to those files, including the prepare script in all package.json

• remove config.js from all apps
• cleanup preview script

How to test

Checklist

• [ ] Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• [ ] Make sure to add/update documentation regarding your changes.
• [ ] You are NOT deprecating/removing a feature.

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