Remove dead rustc_allowed_through_unstable_modules for std::os::fd contents
As far as I was able to reconstruct, the history here is roughly as follows:
- https://github.com/rust-lang/rust/pull/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 https://github.com/rust-lang/rust/issues/99502.)
- Shortly thereafter, https://github.com/rust-lang/rust/pull/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, https://github.com/rust-lang/rust/pull/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.
e1e26f339d