Retire support for "haskellPackageOverrides" from the Nixpkgs config file.

54baa53df1 added support for the special
attribute "haskellPackageOverrides" to the Nixpkgs config file to which users
could add their Haskell-specific package overrides. That mechanism has since
then been replaced by the more general "packageOverrides", which has
subsequently been replaced by Nixpkgs overlays, which have recently been
extended for Haskell-specific needs by the "haskell.packageOverrides"
attribute.

"haskellPackageOverrides" was never documented anywhere, so it seems unlikely
that its removal is going to affect anyone.

pkgs/top-level/all-packages.nix

@@ -7080,7 +7080,7 @@ in
   haskell = callPackage ./haskell-packages.nix { };
 
   haskellPackages = dontRecurseIntoAttrs (haskell.packages.ghc864.override {
-    overrides = config.haskellPackageOverrides or haskell.packageOverrides;
+    overrides = haskell.packageOverrides;
   });
 
   inherit (haskellPackages) ghc;