fieldsets don't work in field admin settings

Oops.

These don't actually work, at least not without ending up with values stored in sub-arrays which is too much hassle to work with (we'd have to update storage from earlier versions, which I don't have time to implement right now.)

Hence removing the fieldset.

Core bug that causes this: #1130946: elements with #tree false that have a parent with #tree TRUE fall to the base of the form values.