Remove async features stream and future for Preview 2

[esoterra]

If I remember correctly and it hasn't changed, wasi-snapshot-preview2 isn't going to rely on async features, but this repo still uses stream and future in its .wit.

[sunfishcode]

Yes. I'm expecting to do that once wit-bindgen supports use and we can start keeping the prototype world in separate files which can be more easily coordinated with the upstream proposal repos.

[esoterra]

That makes sense, just wanted to make sure it was on your radar

[sunfishcode]

WebAssembly/wasi-proposal-template#2 updates the wit-bindgen; if that's accepted, the next step is to start updating all the proposal repos.