Remove Config::stderr

1. It captured stdout and not stderr
2. It isn't used anywhere
3. All error messages should go to the DiagnosticOutput instead
4. It modifies thread local state

src/librustdoc/lib.rs

@@ -691,7 +691,6 @@ fn main_args(at_args: &[String]) -> MainResult {
     rustc_interface::util::run_in_thread_pool_with_globals(
         options.edition,
         1, // this runs single-threaded, even in a parallel compiler
-        &None,
         move || main_options(options),
     )
 }