Fix slang-compilation by avoiding SV_VertexID in one pixel-stage

[fabian-lunarg]

• surfaces with 1.4.350.0/x86_64/bin/slangc (2026.8)
• trivially fixed by removing the unused uint vertexIndex: SV_VertexID

error[E30702]: system value semantic 'SV_VertexID' cannot be used as input in 'pixel' shader stage
  -->.../Vulkan-Samples/shaders/fragment_shader_barycentric/slang/object.frag.slang:27:126
   |
27 | float4 main(VSOutput input, float3 baryCoords: SV_Barycentrics, noperspective float3 baryCoordsAffine: SV_Barycentrics, uint vertexIndex: SV_VertexID, uniform int type)

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