Reduce special-casing for nullary unboxed tuple

When we built the kind of a nullary unboxed tuple, we said, in
TysWiredIn.mk_tuple:

    res_rep | arity == 0 = voidRepDataConTy
                  -- See Note [Nullary unboxed tuple] in Type
            | otherwise  = unboxedTupleRepDataConTy

But this is bogus.  The Note deals with what the 'unarise' transformation
does, and up to that point it's simpler and more uniform to treat
nullary unboxed tuples the same as all the others.

Nicer now.  And it fixes the Lint error in Trac #12115