Checkout: Block removal of Private Registration from cart only when renewing

[gziolo]

Fixes #1410.

Steps to reproduce:

1. Go to http://calypso.localhost:3000/domains/manage.
2. Select a Site.
3. Select a Domain without Private Registration.
4. Navigate to Contacts and Privacy page.
5. Navigate to Privacy Protection page.
6. Click Add Privacy Protection button.
7. Checkout page will load and you won't be able to remove this upgrade from the cart (remove icon is missing):

Testing

1. Do (1)-(6) from steps to reproduce.
2. You should be able to remove Private Registration from the cart.
3. Go to http://calypso.localhost:3000/purchases.
4. Select a Domain purchase with Private Registration that is expiring.
5. Click Renew button.
6. You shouldn't be able to remove Private Registration from the cart. Domain needs to be renewed together with Private Registration.

[gziolo]

@scruffian @fabianapsimoes: Can you confirm it doesn't break anything?

[gziolo]

This function fits much better in cart related logic.

[drewblaisdell]

This works properly and improves the quality of the code it touches. I tested both domain cases. LGTM.

[gziolo]

Thanks @drewblaisdell for review! :)