Clean up superfluous explicit import

Current nightly considers use nix::self as an unused import as nix is
already through the extern prelude (since edition 2018) and generates a
warning. Remove the explicit import to silence it.

See:

* Output of 'cargo --verbose check'
* https://doc.rust-lang.org/stable/reference/names/preludes.html#extern-prelude

src/posix/tty.rs

@@ -5,7 +5,7 @@ use std::time::Duration;
 use std::{io, mem};
 
 use nix::fcntl::{fcntl, OFlag};
-use nix::{self, libc, unistd};
+use nix::{libc, unistd};
 
 use crate::posix::ioctl::{self, SerialLines};
 use crate::posix::termios;