One more fix to updates of the "Slice now" and "Export G-code buttons"

- "Sliced now" button is hidden when "Background process" is selected
- deleted extra enable_buttons() function call

src/slic3r/GUI/GUI_ObjectList.cpp

@@ -263,7 +263,6 @@ void ObjectList::selection_changed()
         event.SetEventObject(this);
         wxPostEvent(this, event);
     }
-    wxGetApp().sidebar().enable_buttons(!m_objects_model->IsEmpty());
 
     part_selection_changed();