From inside your course, select the "Course Admin" button

On the Course Administration page, select the "Assignments" button.

On the Assignment Submission Folders page,  scroll to find the quiz you want to disconnect from the grade item and select the chevron drop-down menu button for that item.

From the drop-down menu that appears, select the "Edit Assignment" button

On the "Edit Assignment" page,  scroll down to find the "Grade Out Of" section and select the drop-down chevron button.

In the drop-down menu, select the "Not In Grade Book" option so this assignment will no longer be associated with any grade item.

Finally select, the "Save and Close" button to save your changes.