Fix warnings in raytrace-parallel example

[ghost]

This fixes two warnings I'm seeing in the raytrace-parallel example when running cargo build --all with nightly-2019-08-23-x86_64-apple-darwin.

[alexcrichton]

Thanks! This warning doesn't actually happen on wasm so perhaps at the top of pool.rs you could put #![cfg_attr(not(target_arch = "wasm32"), allow(dead_code))]?

(with a comment indicating why it's there)

[ghost]

@alexcrichton thanks. I've made the requested change along with a comment.

[alexcrichton]

👍