Revert "Implement AsRawFd/IntoRawFd for RawFd"

This reverts commit 2cf686f2cdd6446a3cd47df0305ead40fabe85df (#40842)

RawFd is a type alias for c_int, which is itself a type alias for i32.
As a result, adding AsRawFd and IntoRawFd impls for RawFd actually adds
them for i32.

As a result, the reverted commit makes this valid:

```
use std::os::unix::io::AsRawFd;

fn arf<T: AsRawFd>(_: T) {}

fn main() {
    arf(32i32)
}
```

Implimenting AsRawFd and IntoRawFd for i32 breaks the promises of both
those traits that their methods return a valid RawFd.

r? @aturon
cc @Mic92 @kamalmarhubi

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