If a license is removed for a database, a banner at the top of the page shows up saying, "You have unlicensed servers. Unlicensed servers will not be monitored and will not raise alerts. Manage licenses | Manage monitored servers". And, on the "Configuration > Monitored servers" page the databases show with a red x, as Unlicensed, and a Manage licenses link.

It is good that licenses can be removed for individual databases, however it would be better to be able to remove the databases once they are unlicensed, so that we don't have to have the banner and the unlicensed message on the monitored servers page.