Blender Git Loki

Cycles: remove disabled kernel_volume_get_final_homogeneous_extinction code.

I'm not quite sure why this would be needed. It gives per color channel sigma,
which we should have so we can compute per channel attentuation without noise
instead of the single float now.

However I think this should be handled at a deeper level by making sigma a
float3 everywhere instead of doing some kind of correction for this afterwards.