How can you drop the witness of a mirroring session when the partners have been dismantled?

The clean way of removing the witness is by issuing this command while connected to the principal:

ALTER DATABASE foo SET WITNESS OFF;

Sometimes however, both partners have been dismantled. In that case this will work:

• Start witness SQL Server in single user mode.
• Connect my instance with an admin connection.
• Delete orphaned entries in sys.syslogshippers.
• Restart SQL in multi user mode.

Source: https://blogs.msdn.microsoft.com/ialonso/2011/12/13/how-can-you-drop-the-witness-of-a-mirroring-session-when-the-partners-have-been-dismantled/