nixos/pantheon: Drop qgnomeplatform

The upstream is not maintained, and qadwaitadecoration
does not seem to be xorg-related. We should have proper
shadows when using SSD.

nixos/doc/manual/release-notes/rl-2311.section.md

···
 
 - The Cinnamon module now enables XDG desktop integration by default. If you are experiencing collisions related to xdg-desktop-portal-gtk you can safely remove `xdg.portal.extraPortals = [ pkgs.xdg-desktop-portal-gtk ];` from your NixOS configuration.
 
-- GNOME module no longer forces Qt applications to use Adwaita style since it was buggy and is no longer maintained upstream. If you still want it, you can add the following options to your configuration but it will probably be eventually removed:
+- GNOME, Pantheon module no longer forces Qt applications to use Adwaita style since it was buggy and is no longer maintained upstream. If you still want it, you can add the following options to your configuration but it will probably be eventually removed:
 
   ```nix
   qt = {

nixos/modules/services/x11/desktop-managers/pantheon.nix

···
         gtk3.out # for gtk-launch program
         onboard
         orca # elementary/greeter#668
-        qgnomeplatform
         sound-theme-freedesktop
         xdg-user-dirs # Update user dirs as described in http://freedesktop.org/wiki/Software/xdg-user-dirs/
       ]) ++ (with pkgs.pantheon; [
···
       programs.bash.vteIntegration = mkDefault true;
       programs.zsh.vteIntegration = mkDefault true;
 
-      # Harmonize Qt applications under Pantheon
-      qt.enable = true;
-      qt.platformTheme = "gnome";
-      qt.style = "adwaita";
+      # Use native GTK file chooser on Qt apps. This is because Qt does not know Pantheon.
+      # https://invent.kde.org/qt/qt/qtbase/-/blob/6.6/src/gui/platform/unix/qgenericunixthemes.cpp#L1312
+      # https://github.com/elementary/default-settings/blob/7.0.2/profile.d/qt-qpa-platformtheme.sh
+      environment.variables.QT_QPA_PLATFORMTHEME = mkDefault "gtk3";
 
       # Default Fonts
       fonts.packages = with pkgs; [