Its simple
1. Delete the windows files on d:
2. Open RUN and type msconfig and delete the entry of windows 8
I can vouch for this method as suggested by dilip..i have done that myself while removing Win 8 DP from my Win 7 & 8 dual boot...so go for it.Easy BCD is the most easier method. Remove Windows 8 from boot loader and delete format D drive while in Windows 7.