1) After connecting to the principal server instance, Right-click the database & go to properties. Select Mirroring from left side option.
2) To remove the witness, delete its server network address from the Witness field.
3) Click Ok to confirm.
You can also perform the same action by below T-SQL.
ALTER DATABASE database_name SET WITNESS OFF
Reference : Rohit Garg (http://mssqlfun.com/)
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