Remove unused imports in std

Author: ILyoan

src/libstd/test.rs

@@ -26,7 +26,6 @@ use core::either::Either;
 use core::either;
 use core::io::WriterUtil;
 use core::io;
-use core::libc::size_t;
 use core::comm::{stream, Chan, Port, SharedChan};
 use core::option;
 use core::prelude::*;