remove unused derives and variants

src/librustdoc/lib.rs

@@ -42,8 +42,6 @@ extern crate test as testing;
 #[macro_use] extern crate log;
 extern crate rustc_errors as errors;
 
-extern crate serialize as rustc_serialize; // used by deriving
-
 use std::default::Default;
 use std::env;
 use std::panic;