Of a half dozen pages on how to remove dual boot Linux (I'm giving an old computer to someone else) only one of them had a sanity check to change the default boot loader first, and make sure it was working (reboot a few times) before deleting the Linux partition. All the rest said to delete Linux first, then try and repaid/fix the boot, which seems quite crazy! The safe article: https://itsfoss.com/uninstall-ubuntu-linux-windows-dual-boot/