Purchases: Domain renewals: Removing private registration

[scruffian]

A user can remove private registration when renewing a domain. We need to prevent this.

@matthusby said:

The way it works now is that you can't just remove it from your cart, the user needs to explicitly remove privacy before going to checkout.

I think this makes sense as removing privacy will expose the domain registrant details so making it a separate action from purchase seems like the best way to handle it.