To delete a field:
1. Hover over the field to reveal the addition controls.
2. Click on the delete icon, which is the third icon.
3. This will delete the selected field.
When deleting a field, make sure that there is no data which has been submitted for that field. If you delete a field for which there is data which has been submitted, you won't be able to see that data anymore under the corresponding collection. For example, if you have the Company field in your Attendee form, and you delete it, you won't be able to see the Company column (and its data) anymore under the Attendees grid.