finish replacing direct3d-headers with directx-headers package

[slimsag]

direct3d-headers is replaced by the newer directx-headers, which is a better approach and long-term sustainable source for d3d headers. We just need to actually use it everywhere.

• direct3d-headers is no longer used anywhere.
• direct3d-headers README has been updated to indicate directx-headers should be used.
• direct3d-headers has been moved to hexops/mach-graveyard