Clean up superfluous import from extern prelude

Our dependencies get passed to rustc with '--extern' (see output of
'cargo --verbose check') and starting with the 2018 edition, they are
automatically provided trough the extern prelude
(https://doc.rust-lang.org/stable/reference/names/preludes.html#extern-prelude).

So no need for declaring the use explicitly here. Current nightly
reports this as unused import and this silences the warning.

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;