From inside your course, select COURSE ADMIN

On the Course Administration page, select Assignments.

On the Assignment Submission Folders page,  scroll to find the quiz you want to disconnect from the grade item and select the chevron.

From the drop-down menu that appears, select EDIT SUBMISSION FOLDER

On the Edit Submission Folder page,  scroll down to find the Grade Item field and select the chevron.

In the drop-down menu, select NONE at the top of the list so this assignment will no longer be associated with any grade item.

Finally select, SAVE AND CLOSE button at the bottom of the screen.