Remove GATHER_TEXTURE3D (does not exist)

Assets/ScriptableRenderLoop/ShaderLibrary/API/D3D11.hlsl

 #define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index) textureName.Gather(samplerName, float3(coord2, index))
 #define GATHER_TEXTURECUBE(textureName, samplerName, coord3) textureName.Gather(samplerName, coord3)
 #define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index) textureName.Gather(samplerName, float4(coord3, index))
-#define GATHER_TEXTURE3D(textureName, samplerName, coord3) textureName.Gather(samplerName, coord3)