Remove dead rustc_allowed_through_unstable_modules for std::os::fd contents

[RalfJung]

As far as I was able to reconstruct, the history here is roughly as follows:

• Allow using stable os::fd::raw items through unstable os::wasi module #99723 added some rustc_allowed_through_unstable_modules to the types in std::os::fd::raw since they were accessible on stable via the unstable std::os::wasi::io::AsRawFd path. (This was needed to fix Regression on nightly when compiling mio for wasm32-wasi #99502.)
• Shortly thereafter, Make std::os::fd public. #98368 re-organized things so that instead of re-exporting from an internal std::os::wasi::io::raw, std::os::wasi::io::AsRawFd is now directly re-exported from std::os::fd. This also made library/std/src/os/wasi/io/raw.rs entirely dead code as far as I can tell, it's not imported by anything any more.
• Shortly thereafter, Mark std::os::wasi::io::AsFd etc. as stable. #103308 stabilizes std::os::wasi::io, so rustc_allowed_through_unstable_modules is not needed any more to access std::os::wasi::io::AsRawFd. There is even a comment in library/std/src/os/wasi/io/raw.rs saying the attribute can be removed now, but that file is dead code so it is not touched as part of the stabilization.

I did a grep for pub use crate::os::fd and all the re-exports I could find are in stable modules. So given all that, we can remove the rustc_allowed_through_unstable_modules (hoping they are not also re-exported somewhere else, it's really hard to be sure about this).

I have checked that std still builds after this PR on the wasm32-wasip2 target.

r? @Mark-Simulacrum

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I moved this to io/tests to match library/std/src/os/unix/io/tests.rs.

@bors r+

📌 Commit f3cf39f has been approved by Mark-Simulacrum

It is now in the queue for this repository.