refactor(nixos): inline module enable conditions

configs/nixos/modules/desktop/session/portals.nix

@@ -1,13 +1,10 @@
 { config, lib, ... }:
 # XDG desktop portal routing for in-app screen sharing and file dialogs.
-let
-  cfg = config.modules.desktop.session.portals; # NOLINT(let_order)
-in
 {
   options.modules.desktop.session.portals.enable =
     lib.mkEnableOption "XDG desktop portals for in-app screen sharing and file dialogs";
 
-  config = lib.mkIf cfg.enable {
+  config = lib.mkIf config.modules.desktop.session.portals.enable {
     # The backend packages arrive with the compositor, so only the routing is set
     # here.
     # Screen sharing, screenshots, and global shortcuts need the compositor's