Remove some unused extern crate

Author: oli-obk

miri/lib.rs

@@ -6,11 +6,8 @@
 // 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};