Fix breakage from previous cherry pick

Author: brson

src/librustc/lib.rs

@@ -32,7 +32,6 @@
 #![feature(clone_from_slice)]
 #![feature(collections)]
 #![feature(const_fn)]
-#![feature(core)]
 #![feature(duration_span)]
 #![feature(dynamic_lib)]
 #![feature(enumset)]

src/libstd/time/duration.rs

@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[cfg(stage0)]
 use prelude::v1::*;
 
 use ops::{Add, Sub, Mul, Div};