Welcome to Eight Forum.
Mostly we can not remove/delete/erase recognizable system partition from within the system environment using native disk management as it is protected.
You can do it through third party disk/partition program such as Partition wizard, Easeus, etc.
Or, using bootable utility disk.
Edit: When you done erasing, you need to remove W8 from boot menu (if it was dual boot) using EasyBCD from W7 environment.
You cannot delete Windows 8 (drive D) as it is Windows System partition - the System partition in Windows has the boot files !
1. Mark Windows 7 partition as active.
2. Reboot to Windows 7 Repair CD/DVD/USB and run StartUp Repair up to 3 times with rebooting.
This is the recomended way on "sevenforums".
Partition - Mark as Active - Windows 7 Forums
Startup Repair - Run 3 Separate Times - Windows 7 Forums
Not sure if you've handled this yet, but here's an easy way how.
Copy bootmgr file from D: to C:
Copy boot folder from D: to C: and skip over the files it tells you it can't copy.
Open an elevated command prompt and run this command:
bcdedit /export C:\boot\bcd
Now set C: to be the active partition in Disk Management. Reboot and now you can format D:
Be sure to also remove the reference to D: in the bcd. You can use EasyBCD for that.
EasyBCD's "Edit Boot Menu" tab has a box to check to "Skip the boot menu".
Why not just clean-up the MBR with EasyBCD.
From what I recall, EasyBCD has an option of "migrating" the Windows boot loader files from one partition to another, and in the process, updates the MBR accordingly.
I think also have the option to Repair the boot files.
So, if the boot loader files are in the Win7 partition, after removing the Win8 partition, you would just use this EasyBCD Repair option and you would be OK again.
Fixed.....Just delete windows 8 CP via windows 7 installer......and fix booting with windows 7 repair tool on windows 7 DVD