Remove the experimental d3d12 Qt Quick backend

There is no point in shipping the experimental D3D12 backend of Qt Quick anymore. It has fewer features than the QRhi-based rendering path, and is likely not used in practice by anyone.

Note that removing the plugin from the qtdeclarative repo is simple, but also the documentation has to be scraped to remove the associated bits. This may involve reworking of certain sections on some of the doc pages. Besides the scenegraph-related documentation pages, there is also a quite big related section in the ShaderEffect docs. We need to carefully remove the HLSL-specific bits while keeping thing that are relevant regardless.