Hi Robert,
By the fact that the 'Apply' button still shows as being active (blued rather than grey), I have the impression that fields in the OBS and Fiscal Periods were at one time used as a filtering criteria. If you removed these as being filtering critera, then failed to 'Apply' the change, the UI will still allow you to open the 'Select Fields' dialog box to remove fields, but it will prevent removals of fields that it still "remembers".
If you return to the Adhoc design page, click the 'Apply' button, then open the 'Select Fields' dialog box once again, you may find that can now remove those fields and folders that you no long wish to use.
Let me know if that works.
Cheers,
Will