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]