nixos/lightdm-tiny-greeter: remove `nixpkgs.config` usage

nixos/modules/services/x11/display-managers/lightdm-greeters/tiny.nix

···
 
     services.xserver.displayManager.lightdm.greeters.gtk.enable = false;
 
-    nixpkgs.config.lightdm-tiny-greeter.conf =
+    services.xserver.displayManager.lightdm.greeter =
     let
       configHeader = ''
         #include <gtk/gtk.h>
···
         static const char *pass_text = "${cfg.label.pass}";
         static const char *session = "${dmcfg.defaultSession}";
       '';
+      config = optionalString (cfg.extraConfig != "") (configHeader + cfg.extraConfig);
+      package = pkgs.lightdm-tiny-greeter.override { conf = config; };
     in
-      optionalString (cfg.extraConfig != "")
-        (configHeader + cfg.extraConfig);
-
-    services.xserver.displayManager.lightdm.greeter =
       mkDefault {
-        package = pkgs.lightdm-tiny-greeter.xgreeters;
+        package = package.xgreeters;
         name = "lightdm-tiny-greeter";
       };
 

pkgs/applications/display-managers/lightdm-tiny-greeter/default.nix

···
 { lib, stdenv, linkFarm, lightdm-tiny-greeter, fetchFromGitHub
-, pkg-config, lightdm, gtk3, glib, wrapGAppsHook, conf ? "" }:
+, pkg-config, lightdm, gtk3, glib, wrapGAppsHook, config, conf ? config.lightdm-tiny-greeter.conf or "" }:
 
 stdenv.mkDerivation rec {
   pname = "lightdm-tiny-greeter";

pkgs/top-level/all-packages.nix

···
 
   lightdm-mobile-greeter = callPackage ../applications/display-managers/lightdm-mobile-greeter { };
 
-  lightdm-tiny-greeter = callPackage ../applications/display-managers/lightdm-tiny-greeter {
-    conf = config.lightdm-tiny-greeter.conf or "";
-  };
+  lightdm-tiny-greeter = callPackage ../applications/display-managers/lightdm-tiny-greeter { };
 
   ly = callPackage ../applications/display-managers/ly { };