[HDRP][DXR] Removed work-around on vertex color in ray tracing, after changes to the engine

[eturquin]

Removed temp fix introduced by: #5268
after modifications to the engine that handle this case in a more robust manner: https://ono.unity3d.com/unity/unity/pull-request/130215
Vertex color (and other vertex-based attributes, like UVs, normals, etc.) are now defaulting to the same value in raster and ray-tracing. In the case of color, it is now defaulting to white instead of black.

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[sebastienlagarde]

please launch DXR test