This removes the exposed stateless variant since the standard variant
has similar speed now.
Using `./benchmark --filter wyhash --count 1024`, the speed change has
changed from:
wyhash
iterative: 4093 MiB/s [6f76b0d5db7db34c]
small keys: 3132 MiB/s [28c2f43c70000000]
to
wyhash
iterative: 6515 MiB/s [673e9bb86da93ea4]
small keys: 10487 MiB/s [28c2f43c70000000]
This is user specified and the user doesn't necessarily have to use one
of the provided polynomials declared hence we can't use an enum.
Thanks @daurnimator for catching this.
