chore: remove unused file electron_gtk.sigs

shell/browser/ui/electron_gtk.sigs

@@ -1,7 +0,0 @@
-GtkFileChooserNative* gtk_file_chooser_native_new(const gchar* title, GtkWindow* parent, GtkFileChooserAction action, const gchar* accept_label, const gchar* cancel_label);
-void gtk_native_dialog_set_modal(GtkNativeDialog* self, gboolean modal);
-void gtk_native_dialog_show(GtkNativeDialog* self);
-void gtk_native_dialog_hide(GtkNativeDialog* self);
-gint gtk_native_dialog_run(GtkNativeDialog* self);
-void gtk_native_dialog_destroy(GtkNativeDialog* self);
-GType gtk_native_dialog_get_type(void);