To uninstall kwin scripts, you must ironically click the button that says 'Get New Scripts' (which is certainly not what I want to actually do), then filter by installed. There should be a button to remove the currently selected script in the kcm list.

I actually came here to report that it's not possible to remove scripts, which I had been intending to do for quite a while. I finally figured out how to do it by accident while I was writing that report. The way it currently works is very unintuitive.