On the gradebook Setup page you can only delete a manual grade item. To do so, click the Edit for that item, and then Delete.
If the grade item is an activity that was created on your course homepage, you must
delete the activity from your course homepage to remove it from the gradebook. From
your course homepage to the activity, click the three dots below, and choose Delete.
If you do not want to delete the activity, but you do want to remove it from the gradebook,
go into the settings of the activity and change the Grade Type to None (Assignment
or Turnitin), set the maximum grade to 0 (Quiz), or for a Forum change the Grade Type
to None or the Aggregate Type to No Ratings (Forum).
Alternatively, on the gradebook Setup page you can check the box for Weight for the
item and enter 0. Save. That grade item will then not be included in the course total.