
quote from google
Once you have deleted the partitions you need to go into the EFI partition (mounting at /boot/efi) and delete the windows folder. Then do a sudo update-grub. Next time you boot it should boot directly into linux with no grub menu. You can then resize your / partition to use all the drive, you need to boot from your install stick and run gparted from that. You can't change a mounted partition and you can't unmount / booting normally. Make sure you have a backup first.