Auto merge of #1461 - newpavlov:patch-4, r=gnzlbg

Remove WASI Core API

Closes #1434

This change does not break the backwards compatibility promise since WASI Core API is unstable right now. If applications or libraries want to use Core API directly they should use [`wasi`](https://crates.io/crates/wasi) instead of `libc`.

Blocked by: rust-lang/rust#63676

cc @sunfishcode

Author: bors