remove no longer needed (and sometimes broken) 'extern crate'

Author: RalfJung

src/bin/miri-rustc-tests.rs

@@ -1,13 +1,9 @@
 #![feature(rustc_private)]
-extern crate getopts;
 extern crate miri;
 extern crate rustc;
-extern crate rustc_codegen_utils;
 extern crate rustc_driver;
-extern crate rustc_errors;
 extern crate rustc_hir;
 extern crate rustc_interface;
-extern crate rustc_metadata;
 extern crate rustc_span;
 
 use std::io;