Cleanup after removal of labels (Pancake)

The labels are now removed from the Pancake syntax, but there are still some residual syntactic redundancy that we should clean up. Namely,

```
Datatype:
word_lab = Word ('a word)
(* labels were defined here *)
End

Datatype:
v = Val ('a word_lab)
| Struct (v list)
End

Overload ValWord = “\w. Val (Word w)”
```

should be just:

```
Datatype:
v = Val ('a word)
| Struct (v list)
End
```