message: remove unneeded backtrace feature
diff: @@ -160,7 +160,7 @@ fn setup(ask_user: bool) {
  File::create(dir.join("Xargo.toml")).unwrap()
  .write_all(br#"
  [dependencies.std]
  -features = ["panic_unwind", "backtrace"]
  +features = ["panic_unwind"]
  [dependencies.test]
  stage = 1
  