[Development] Apparently dead code: GPU_BLACKLIST and friends

[Edward Welbourne]

I wrote:
> I notice that QTest includes support, in [0], alongside its BLACKLIST
> files for problematic tests, for a GPU_BLACKLIST file; [...]
>
> Inevitably, I'm contemplating a featurectomy.

The reviews can be found here:

https://codereview.qt-project.org/#/q/status:open+branch:dev+topic:gpu-blacklist,n,z

	Eddy.