Remove useless `then` keyword from grammar.

It is best not to add too many keywords to the language, especially when the grammar does not need them.

```haskell
i (Arg1) =
    if not Arg1 false else false end
```

![8](https://cloud.githubusercontent.com/assets/278727/2930195/005f076c-d793-11e3-8118-d63b41ff0997.png)