I want to COMPLETELY remove Windows.

I tried downloading Windows via Bootcamp, without knowing that I had to pay for Windows' activation. Afterwards, I deleted Bootcamp, and thought that Windows would be gone from my Macbook Pro. However, whenever I start-up my laptop, after shutting it down, it shows a screen from Windows that alerts me that my laptop requires a repair, when I'm well aware that it doesn't, and I can just switch to macOS, using the " immediately hold option " to switch to macOS. I want to remove Windows entirely, because I find it very irritating that I have to take so much longer to get my laptop running, on macOS.