It doesn't have to be removed per se, but at least I'd like to make it not appear on the form. I noticed that it's part of the mandatory fields (ie. first name, last name etc.) so I can't just select "OFF" just like I do to custom fields.
In the main Configuration area of DT Register, there is a "Fields" tab where you can set to Hide / Show any of the default fields. You can set your preferences for free events, paid events, and also for each member of a group registration.