nixos/ibus: Do not force ibus input method module on GTK

The environment variable is meant for debugging and GTK should already choose the correct input method module automatically. For example, GTK 4 will use the `ibus` module on X11 and `wayland` module on Wayland.

It was introduced in f222abea44.

Other distros like [Ubuntu back in 2020](https://discourse.ubuntu.com/t/ibus-no-more-gtk-im-module-ibus/17727) recognised that setting this was not beneficial and instead lead to more issues, e.g. crashes in ibus.

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

@@ -63,7 +63,7 @@ in
         default = false;
         description = ''
           Use the Wayland input method frontend.
-          This doesn't set `GTK_IM_MODULE` and `QT_IM_MODULE` environment variables.
+          This doesn't set `QT_IM_MODULE` environment variable.
           See [Using Fcitx 5 on Wayland](https://fcitx-im.org/wiki/Using_Fcitx_5_on_Wayland#GTK_IM_MODULE).
         '';
       };
@@ -90,7 +90,6 @@ in
       XMODIFIERS = "@im=ibus";
     }
     // lib.optionalAttrs (!cfg.waylandFrontend) {
-      GTK_IM_MODULE = "ibus";
       QT_IM_MODULE = "ibus";
     };