Mailchimp reported as connected after disconnecting Jetpack

#### Steps to reproduce the issue

1. Connect, then disconnect Jetpack
2. Make a request to `/wp-json/wpcom/v2/mailchimp`
3. Observe that the response states Mailchimp is connected
4. Observe the `connect_url` link in the response directing to a blank page on wordpress.com

#### What I expected

Expected to see an `unavailable_site_id` error, as it is before connecting Jetpack.

Start by reproducing the issue with a request to /wp-json/wpcom/v2/mailchimp after connecting and disconnecting Jetpack, then trace the endpoint's Mailchimp connection response. Done means the disconnected state returns an unavailable_site_id error and no longer provides a connect_url leading to a blank wordpress.com page.

Written by the indexing model from the issue text.