Hi
You can also use EasyBCD to make sure that the computer boots directly into Windows 10 without looking for Windows 7.
The free versions at the bottom of the page...
EasyBCD
If you delete Windows 7 while it's still in the boot list your computer won't boot, (I found this out the hard way years ago). LOL
Use EasyBCD to set Windows 10 as the default OS, remove Windows 7 from the boot list, and tell it not to show the boot menu,
Once you reboot and get it to boot directly to Windows 10, it's then safe to remove the Windows 7 installation.
Don't format the hard drive or remove the boot sector, if the boot files are on that drive that would screw things up as well.
Just delete the Windows install and related files, i.e the...