Redox is unix-family

Remove unnecessary `any(target_os = "redox", unix)`, change to just `unix`.
[This](https://github.com/BurntSushi/same-file/blob/master/src/lib.rs#L85) used to be necessary, but in 2019 [Redox became unix-family](https://github.com/rust-lang/rust/pull/60547).

