Runtime removal: fully remove rtio

This patch cleans up the remnants of the runtime IO interface.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.

src/libstd/sys/unix/mod.rs

@@ -9,6 +9,11 @@
 // except according to those terms.
 
 #![allow(missing_doc)]
+#![allow(non_camel_case_types)]
+#![allow(unused_imports)]
+#![allow(dead_code)]
+#![allow(unused_unsafe)]
+#![allow(unused_mut)]
 
 extern crate libc;