So you went ahead and dual booted the Windows 8 Developer Preview with your Windows 7 machine, but decided you don’t want it anymore. There are lots of instructions online for how to remove it when you are using a standard boot, but if you are using UEFI your life is much simpler. (I’m not going into detail since if you figured out how to dual boot, you probably can handle a partition or two)
Using Disk Management, delete the volume for your windows 8 installation. Optionally, expand the volume for your Windows 7 install to get the space back.
If you want to speed up the boot process a bit, Open the Advanced System Settings (rt click Computer, Properties, Advanced System Settings), click Settings… under Startup and recovery, and make sure the check box next to “Time to display list of operating systems” is unchecked, (and that Windows 7 is the default operating system).
All done! No need to edit the BCD when using EFI boot, it appears that windows automatically removes the windows 8 entry when you delete the partition. (I ran bcdedit just to check)