Force usage of a worker thread for stdin on all platforms (#7058)

This commit is a follow-up to #6833 to remove the `unix` module for
handling stdio which sets stdin to nonblocking mode. I've just now
discovered that on macOS at least configuring `O_NONBLOCK` for stdin
affects the stdout/stderr descriptors too. This program for example will
panic:

    fn main() {
        unsafe {
            let r = libc::fcntl(
                libc::STDIN_FILENO,
                libc::F_SETFL,
                libc::fcntl(libc::STDIN_FILENO, libc::F_GETFL) | libc::O_NONBLOCK,
            );
            assert_eq!(r, 0);
        }

        loop {
            println!("hello");
        }
    }

It was originally assumed that updating the flags for stdin wouldn't
affect anything else except Wasmtime, but because this looks to not be
the case this commit removes the logic of registering stdin raw with
Tokio and instead unconditionally using the worker thread solution which
should work in all situations.

crates/wasi/src/preview2/stdio.rs

@@ -8,14 +8,7 @@ use crate::preview2::{HostOutputStream, OutputStreamError, WasiView};
 use bytes::Bytes;
 use is_terminal::IsTerminal;
 
-#[cfg(unix)]
-mod unix;
-#[cfg(unix)]
-pub use self::unix::{stdin, Stdin};
-
-#[allow(dead_code)]
 mod worker_thread_stdin;
-#[cfg(windows)]
 pub use self::worker_thread_stdin::{stdin, Stdin};
 
 // blocking-write-and-flush must accept 4k. It doesn't seem likely that we need to