If you ever need to delete an NTFS partition from DOS, the Windows NT 3.1 Resource Kit has the solution.  Download the kit, extract the DELPART.EXE utility to a DOS boot disk, boot from the disk and then run the utility.  It's self-explanatory from there.

I wonder why this utility is no longer in the Resource Kits?

Note:  I kept getting a “Page Not Found” error while trying to download the resource kit, but it eventually worked.