Simplify hessian_tensor_product, removing numpy dependency

[j-towns]

No description provided.

[j-towns]

The test failures appear to have been caused by an issue with Travis/Github.

[mattjj]

Nice simplification! But should we just remove this function? Your revision makes clear that it's redundant with make_hvp.

[j-towns]

Hmm, I can think of at least one downstream project that likes to use hessian_vector_product, maybe a compromise would be to re-implement this as a wrapper around make_hvp (not sure why I didn't already do that).