Remove extraneous ?s in struct grammar
The visibility grammar already has `EMPTY` as an option.
>
> _StructField_ :
> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup>
-> &nbsp;&nbsp; [_Visibility_]<sup>?</sup>
+> &nbsp;&nbsp; [_Visibility_]
> &nbsp;&nbsp; [IDENTIFIER] `:` [_Type_]
>
> _TupleFields_ :
>
> _TupleField_ :
> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup>
-> &nbsp;&nbsp; [_Visibility_]<sup>?</sup>
+> &nbsp;&nbsp; [_Visibility_]
> &nbsp;&nbsp; [_Type_]
A _struct_ is a nominal [struct type] defined with the keyword `struct`.
