Cannot drop the database because it is being used for replication

The database used to be part of a replication but apparently something remained somewhere else.

The following commands helped:

use master
go

exec sp_helpreplicationdb 
exec sp_removedbreplication 'Northwind'

ALTER DATABASE Northwind SET SINGLE_USER WITH ROLLBACK IMMEDIATE 
ALTER DATABASE Northwind SET MULTI_USER

DROP DATABASE Northwind