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

stack.yaml

@@ -6,13 +6,6 @@ packages:
 extra-deps:
 - X11-1.10
 
-flags:
-  xmonad:
-    # stack doesn't support automatic flags
-    # https://cabal.readthedocs.io/en/latest/cabal-package.html#resolution-of-conditions-and-flags
-    # https://github.com/commercialhaskell/stack/issues/1313#issuecomment-157259270
-    quickcheck-classes: false
-
 nix:
   packages:
     - zlib