nixos/iotop: don't install the package globally

The binary will be in `/run/wrappers/bin` and adding `pkgs.iotop` won't
have any effect.

See also https://github.com/NixOS/nixpkgs/pull/51749#discussion_r254724170

nixos/modules/programs/iotop.nix

@@ -9,7 +9,6 @@ in {
     programs.iotop.enable = mkEnableOption "iotop + setcap wrapper";
   };
   config = mkIf cfg.enable {
-    environment.systemPackages = [ pkgs.iotop ];
     security.wrappers.iotop = {
       source = "${pkgs.iotop}/bin/iotop";
       capabilities = "cap_net_admin+p";