Non-existant variable in error message in type family instance
## Summary
In an error message about unused variables in type family instance, GHC mentions a variable, that doesn't exist in source code.
## Steps to reproduce
```haskell
{-# LANGUAGE TypeFamilies, ExplicitForAll, PolyKinds #-}
module Bug where
type family F
type instance forall a. F = ()
```
This code produces the following error message:
```
• Type variables ‘k’, ‘a’ are bound by a forall,
but not used in the family instance
```
What is `k`? Why GHC mentions it?
## Expected behavior
`Type variable ‘a’ is bound by a forall...`
## Environment
* GHC version used:
- Versions [8.8 .. HEAD] report ‘k’, ‘a’
- 8.6 cannot parse this syntax.
issue