only use #[no_core] in libcore

src/libsyntax/ext/log_syntax.rs

@@ -8,9 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use core::io::WriterUtil;
-use core::prelude::*;
-
 use ast;
 use codemap;
 use ext::base::*;