Fix #16299 by deleting incorrect code from IfaceSyn

GHCi's `:info` command was pretty-printing Haskell98-style data types
with explicit return kinds if the return kind wasn't `Type`. This
leads to bizarre output like this:

```
λ> :i (##)
data (##) :: TYPE ('GHC.Types.TupleRep '[]) = (##)
        -- Defined in ‘GHC.Prim’
```

Or, with unlifted newtypes:

```
λ> newtype T = MkT Int#
λ> :i T
newtype T :: TYPE 'IntRep = MkT Int#
        -- Defined at <interactive>:5:1
```

The solution is simple: just delete one part from `IfaceSyn` where
GHC mistakenly pretty-prints the return kinds for non-GADTs.