Can't remove GNU Grub after deleting partition (Not GrubRescue!)

This is really urgent as I need Ubuntu for work.

I installed Ubuntu last year as a partition to dual boot with Windows 10. Then I realized that I didn't designate enough storage space to the Ubuntu partition (only 12gb!); How stupid! Anyway, I then decided to reinstall Ubuntu on my device.

So I deleted my current 12GB Ubuntu partition using Windows diskmgnt.

Then I went through the same process as before, I installed Ubuntu desktop as an ISO file, then used Rufus to flash it onto my USB, then I booted from my USB, when I was greeted with an odd message.

initramfs: Unable to find a medium containing a live file system

I looked into this and quickly realized that I still had a boot option called 'Ubuntu' even though I deleted the partition. This wasn't normal Ubuntu Desktop, it was a minimal GNU Grub.
GNU Grub

I'm not entirely sure that the fact I can not install Ubuntu is to do with this, but it would be nice to remove it once and for all.

Can anyone please help me? I need Ubuntu for work and I'm not in a position to go out and buy a new PC.

Thank you so much for understanding, and sorry if I sound a bit uptight as this is my first post.