Domain credit prompt showing for site that already has a mapped domain

Expected behavior

If a site already has a mapped custom domain, I don't expect to see the domain credit prompt on the site detail screen.

Actual behavior

The domain credit prompt shows up, even if the site has a mapped domain.

Steps to reproduce the behavior

The prompt also shows up for sites that use .blog subdomains. Create a site with a .blog subdomain and add a plan to it. You should see the domain credit prompt.

We could potentially hide the prompt using DomainsService's refreshDomainsForSite method, as this tells us what sort of domains a site has.

However, Calypso also appears to have the same behaviour. And I can see that it's a valid case for a user with a .blog subdomain (or even an older custom mapped domain) that they might want to register a new domain.

Perhaps instead, a better solution would be to provide a way to dismiss the domain credit prompt if you're not interested (although would we need somewhere for the user to find it later?).

Tested on iPhone 11 Pro, iOS 13.1, WPiOS 13.3

Start by tracing the site detail screen's domain credit prompt and the DomainsService refreshDomainsForSite path, then compare behavior for mapped domains and .blog subdomains. The issue does not settle whether to hide the prompt or add dismissal, so confirm that requirement first; done means the chosen behavior is covered for the reported cases.