nixos/ibus: do not default-install ibus-qt

ibus-qt has not seen a release in 5 years and is only relevant for Qt
4.x, which is becoming more and more rare. Using my current laptop as a
data point, ibus-qt is the only dependency left that drags in qt-4.8.7.

nixos/modules/i18n/input-method/ibus.nix

@@ -55,7 +55,7 @@ in
 
     # Without dconf enabled it is impossible to use IBus
     environment.systemPackages = with pkgs; [
-      ibus-qt gnome3.dconf ibusAutostart
+      gnome3.dconf ibusAutostart
     ];
 
     environment.variables = {