Mailchimp connected in API response 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.

Reproduce the issue by connecting and disconnecting Jetpack, then request `/wp-json/wpcom/v2/mailchimp` and inspect the response and `connect_url`. Trace the API entry point responsible for the Mailchimp response; done means the disconnected state returns an `unavailable_site_id` error instead of reporting Mailchimp as connected.

Written by the indexing model from the issue text.