backends/virtual: Drop KWIN_WAYLAND_VIRTUAL_SCREENSHOTS

Virtual backend is used primarily by tests, there are plans to use it for virtual outputs too. But in either case we don't need screenshots.

I also can't imagine how it can be useful as a debugging tool. It's more convenient to spin a windowed kwin instance.

The main motivation behind this change is to get rid of custom code in virtual output layers and simplify it a bit more.