message: Remove some unused `extern crate`
diff: // From rustc.
  #[macro_use]
  extern crate log;
  -extern crate log_settings;
  #[macro_use]
  extern crate rustc;
  -extern crate rustc_const_math;
  -extern crate rustc_data_structures;
  extern crate syntax;
  use rustc::ty::{self, TyCtxt};
  