At the principal, run:
-- This should tell the witness that it is no longer needed. Might have just had issues with the SSMS GUI.
ALTER DATABASE [DatabaseName] SET SAFETY OFF
-- Here is the other option. I am going to guess that this statment would fail if mirroring has already been disconnected. However, if the above doesn't work, it is worth an attempt.
ALTER DATABASE [DatabaseName] SET PARTNER OFF