Registration questions may need to be removed from your event for a variety of reasons. Questions can either be fully deleted or simply made "admin portal only" (hidden to the public).
To prevent data loss, you cannot delete a question which has been answered by attendees.
Deleting a question
Deleting a question from your event is only possible if no one has answered the question. Furthermore, you are only deleting the question from your event, not the entire system:
Go to Website > Content > Registration pages
Click to "Edit" the page which contains your registration question
Hover over the 'edit' icon and select "Delete"