You said:
”I can't find anywhere how to remove or even edit this discount percentage.”
I have found the editable fields for these discount percentages on both the app and the desktop. On which platform are you having a problem?
Assuming you have worked out how to edit the discount, try this in a desktop browser:
1) Clear your browser cache and refresh the page
2) Change the discount to 20%, save it, verify that it is changed by simulating a booking, and then set it back to 0%. Simulate the booking again.
The reason I suggest doing this in a desktop browser is that sometimes the Airbnb app does not refresh setting changes. It retains the old state, sometimes for a significant period of time. So testing out changes is more reliable in a cache-cleared, desktop browser.