If you go to delete a grade item and the Delete page will not allow you to select the particular grade item you want to delete, that is because it is associated with another item in your course (a quiz, a discussion, or an assignment for example).

Resolution: To delete the grade item,  you will first need to edit the item it's associated with and remove the grade item association from it. 

For example, if you need to delete a quiz grade item that's associated to a quiz, you will need to go to Assessments > Quizzes, edit the quiz, click the Assessment tab, change the Grade Item selector to "None", and click Save and Close.

Afterwards, if you go to Grades and try to delete again, you will be able to delete the grade item.