Remove --mode browser

[havocp]

There's an anaconda-project prepare --mode=browser which uses an HTML UI to ask for missing requirements. The original idea was to have a standard UI so that plugins could extend it with custom elements, then embed this UI in navigator, etc. However, the way we're using Anaconda Project right now makes this a bad approach (often our UI isn't on the same machine as the project), so we can delete a bunch of code.