Remove D3D9 support, to make future changes easier

[hrydgard]

Fixes #19949

Will merge after 1.19.

[Felipefpl]

Could you release a last official build before deleting dx9? This way ppl with old computers will have a build for reference when dealing with dx9. ;)

[hrydgard]

Well, I guess I can hold off on this until 1.19, since my planned graphics work won't likely happen until then anyway.

[Panderner]

Older DirectX 10 GPUs supports D3D11 backend but it's feature level is limited to 10_0.

[hrydgard]

That would be fine to support though, just like we previously supported feature level 9 for Windows Phone.

[hrydgard]

Note to self: For reference 5ee9cfe removed the D3D9 feature level support, but left in place feature level D3D10 from what I can tell. I don't think we use any functionality that's exclusive to D3D11, really.

[Panderner]

Does Windows Phones support D3D9?

[hrydgard]

No, but they supported d3d11 with feature level 9.