remove unused extern crate

Author: RalfJung

src/lib.rs

@@ -6,13 +6,12 @@
 extern crate log;
 
 // From rustc.
+extern crate syntax;
 #[macro_use]
 extern crate rustc;
 extern crate rustc_data_structures;
 extern crate rustc_mir;
 extern crate rustc_target;
-extern crate rustc_driver;
-extern crate syntax;
 
 use std::collections::HashMap;
 use std::borrow::Cow;