qt6: remove stdenv fully (#391178)

pkgs/kde/default.nix

···
       ) allUrls;
     in
     (
-      # This removes reference to deprecated `qt6Packages.stdenv`
-      # so when a KDE package asks for a stdenv, it won't trigger
-      # an eval warning
-      (lib.removeAttrs qt6Packages [ "stdenv" ])
+      qt6Packages
       // frameworks
       // gear
       // plasma

pkgs/top-level/qt6-packages.nix

···
 
   xwaylandvideobridge = kdePackages.callPackage ../tools/wayland/xwaylandvideobridge { };
   });
-} // lib.optionalAttrs config.allowAliases {
-  # when removing, don't forget to remove a workaround in `pkgs/kde/default.nix`
-  stdenv = lib.warn "qt6Packages.stdenv is deprecated. Use stdenv instead." stdenv; # Added for 25.05
 }