https://www.tenforums.com/tutorials/148810-how-remove-uninstall-windows-7-8-10-dual-boot-pc.html

If you have more than one operating system installed on your PC, you will see a Choose an operation system (Windows Boot Manager) screen at startup with the name (description) of each installed OS (boot loader entry) listed on the boot options menu. This tutorial will show you how to remove and uninstall Windows 7, Windows 8, or Windows 10 from a dual boot computer with another Windows operating system installed.