Deleting Custom Fields
Account, Contact, and Deal Custom Fields
To delete a custom field that has been added to either an Account, Contact, or Deal, simply navigate to the Admin/Form Configuration page. Then, make sure you select the tab for the item you want to remove the custom field from (i.e., Account, Contact, or Deal). Then, click on the 3 dots menu next to the custom field you want to delete. Finally, click on "Delete" in the options menu.
Work Item Custom Fields
The process for deleting a custom field that has been added to a work item is the same as above. However, you'll need to go to the Admin/Work Item Types page, then select the type you want to remove the field from.