types: remove references to new packageSet

Reverts part of a04a7272aa38ada45e694281071b720d4abbd0df

packageSet is slated to be introduced in #11866 but currently causes
evalation errors.

/cc @ts468

nixos/modules/services/x11/window-managers/xmonad.nix

···
     services.xserver.windowManager.xmonad = {
       enable = mkEnableOption "xmonad";
       haskellPackages = mkOption {
-        type = lib.types.packageSet;
         default = pkgs.haskellPackages;
         defaultText = "pkgs.haskellPackages";
         example = literalExample "pkgs.haskell.packages.ghc784";

nixos/modules/system/boot/kernel.nix

···
   options = {
 
     boot.kernelPackages = mkOption {
-      type = types.packageSet;
       default = pkgs.linuxPackages;
       # We don't want to evaluate all of linuxPackages for the manual
       # - some of it might not even evaluate correctly.