Remove references to unknown feature io

Author: varkor

src/test/run-pass/issue-13304.rs

@@ -11,8 +11,6 @@
 // ignore-cloudabi no processes
 // ignore-emscripten no processes
 
-#![feature(io)]
-
 use std::env;
 use std::io::prelude::*;
 use std::io;

src/test/run-pass/issue-14456.rs

@@ -11,8 +11,6 @@
 // ignore-cloudabi no processes
 // ignore-emscripten no processes
 
-#![feature(io)]
-
 use std::env;
 use std::io::prelude::*;
 use std::io;

src/test/run-pass/issue-17322.rs

@@ -10,7 +10,7 @@
 
 // pretty-expanded FIXME #23616
 
-#![feature(box_syntax, io)]
+#![feature(box_syntax)]
 
 use std::io::{self, Write};
 

src/test/run-pass/issue-4333.rs

@@ -10,8 +10,6 @@
 
 // pretty-expanded FIXME #23616
 
-#![feature(io)]
-
 use std::io;
 
 pub fn main() {