delete some dead code (#669)

dead code detected via [dead](https://github.com/asottile/dead)

- **`KEYWORDS`**: introduced (unreferenced) in e74117f1
- **`FLOW_CONTROL`**: last referenced in e9a940d6

"clean" command:

```
dead --exclude '^(tests/data/|docs/conf.py|blib2to3/)' | grep -Ev '^(visit_.*|show|_stop_signal|lib2to3_unparse) '
```