Check for unused type variables in type instances is buggy for associated type defaults
Currently, a program like
```haskell
type family F
type instance forall a. F = ()
```
will give rise to an error
```
* Type variable `a' is bound by a forall,
but not used in the family instance.
* In the type instance declaration for `F'
```
Associated type defaults go through the same logic (`checkValidClass` calls `checkValidTyFamEqn` which calls `checkFamPatBinders`), but I think it is passed incorrect arguments, which means that the following program:
```haskell
class C where
type H
type forall c. H = ()
```
does not give rise to any errors, when we would expect a similar error as above.
I will fix as part of my investigations into #23734.
issue