Remove F821 monkeypatch

See python/typeshed#14357 for background.

It seems that pyflakes 3.4.0 changed the F821 check, which we moneypatch. When upgrading pyflakes, we get new errors for the following pattern:

```
X: Foo

a = X.bar  # F821 undefined name `X`
```

We probably need to update our monkeypatch.