Ryan Scott authored Jul 30, 2019 and Marge Bot committed Aug 02, 2019

GHC had an ad hoc validity check in place to rule out pattern
variables bound by type synonyms, such as in the following example:

```hs
type ItemColID a b = Int  -- Discards a,b

get :: ItemColID a b -> ItemColID a b
get (x :: ItemColID a b) = x :: ItemColID a b
```

This hack is wholly unnecessary nowadays, since OutsideIn(X) is more
than capable of instantiating `a` and `b` to `Any`. In light of this,
let's rip out this validity check.

Fixes #17007.