core: Fix fallout of changing #[deriving(Clone)]

src/libcore/result.rs

@@ -276,7 +276,6 @@
 
 #![stable]
 
-use clone::Clone;
 use cmp::PartialEq;
 use std::fmt::Show;
 use slice;

src/libcore/str.rs

@@ -19,7 +19,6 @@
 use mem;
 use char;
 use char::Char;
-use clone::Clone;
 use cmp;
 use cmp::{PartialEq, Eq};
 use default::Default;