There are couple of reasons:
- Volume shader on hair does behave really weird anyway and it's
not something considered a bug really.
- Volume BVH traversal were only used by camera-in-volume check,
which doesn't really make sense to take hair into account since
it'll be rendered wrong anyway.
Such a removal makes both code easier to extend further (as in,
no need to worry about those traversal for hair bvh) and also
reduces stress on GPU compilers.
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