Fix #23007

Previously, data family instances would be treated differently from
data type definitions wrt. unused type variables/patterns:
```haskell
newtype Const a b = Const a
data family Const' a b
newtype instance Const' a b = Const' a
```

The above will warn for `Const'` but not for `Const` because of `b`
isn't mentioned on the RHS.

This leads to the awkward situation, where if you fix it and make it `_b`,
the type becomes the following:
```haskell
:t Const'
Const' :: a -> Const' a _b
```

This is suboptimal.