syntax: Remove use of pub use globs

`quote_expr!` now injects two more (priv) `use` globs.
This may cause extra unused_imports warning.

src/libsyntax/lib.rs

@@ -48,6 +48,7 @@ pub mod util {
 pub mod syntax {
     pub use ext;
     pub use parse;
+    pub use ast;
 }
 
 pub mod owned_slice;