Removed the image setting for gtk buttons

As we do not use buttons any more

src/pdfpc.vala

@@ -138,10 +138,6 @@ namespace org.westhoffswelt.pdfpresenter {
             Gdk.threads_init();
             Gtk.init( ref args );
 
-            // Needed to show the images in the overview
-            var default_settings = Gtk.Settings.get_default();
-            default_settings.set("gtk_button_images", true);
-
             // Initialize the application wide mutex objects
             MutexLocks.init();