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

compiler/rustc_driver/src/lib.rs

@@ -226,7 +226,6 @@ fn run_compiler(
         output_dir: odir,
         file_loader,
         diagnostic_output,
-        stderr: None,
         lint_caps: Default::default(),
         parse_sess_created: None,
         register_lints: None,