gtk3: remove obsolete gobject-introspection from buildInputs

Since #200337, it is propagated from nativeBuildInputs.

pkgs/development/libraries/gtk/3.x.nix

···
   ];
 
   buildInputs = [
-    gobject-introspection
     libxkbcommon
     (libepoxy.override { inherit x11Support; })
     isocodes