ci: Drop support for GHC 8.4

Debian stable is not on 8.6, which was always our guide as to which GHC
versions we want to support.

In particular, this lets us get rid of all the quickcheck-classes
special treatment.

Related: 400730fe3b

tests/Properties/Stack.hs

@@ -1,9 +1,5 @@
-{-# LANGUAGE CPP #-}
 {-# LANGUAGE ScopedTypeVariables #-}
-
-#ifdef VERSION_quickcheck_classes
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
-#endif
 
 module Properties.Stack where
 
@@ -15,13 +11,11 @@ import qualified XMonad.StackSet as S (filter)
 
 import Data.Maybe
 
-#ifdef VERSION_quickcheck_classes
 import Data.Proxy
 import Test.QuickCheck.Classes (
     Laws (lawsTypeclass, lawsProperties), Proxy1 (Proxy1),
     foldableLaws, traversableLaws,
     )
-#endif
 
 
 -- The list returned by index should be the same length as the actual
@@ -65,7 +59,6 @@ prop_differentiate xs =
     where _ = xs :: [Int]
 
 
-#ifdef VERSION_quickcheck_classes
 -- Check type class laws of 'Data.Foldable.Foldable' and 'Data.Traversable.Traversable'.
 newtype TestStack a = TestStack (Stack a)
     deriving (Eq, Read, Show, Foldable, Functor)
@@ -82,6 +75,3 @@ prop_laws_Stack = format (foldableLaws p) <> format (traversableLaws p)
     p = Proxy :: Proxy TestStack
     format laws = [ ("Stack: " <> lawsTypeclass laws <> ": " <> name, prop)
                   | (name, prop) <- lawsProperties laws ]
-#else
-prop_laws_Stack = []
-#endif