Sql Server 2016 Cannot drop database (it is really, really, really stuck)
This issue is pretty tricky to reproduce so I can suggest to try two more solutions, but both require SQL Server to be restarted:


Start SQL Server in single user mode (https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/start-sql-server-in-single-user-mode)
(just add SQL Server service startup parameter -m and restart the service). Then using SQL CMD connect to the SQL instance using local administrator account and try to drop database using DROP DATABASE statement. Perhaps this time you won't get an error related to exclusive access acquisition. 
Another way (pretty primitive I know! But it might be effective either!) is to stop SQL service and delete (or change location) of database files. Then when you start SQL Server back this database should come up in 'recovery pending' status and you should be able to remove it.