Update `remove_demo` so it works

Replaced `ReduxFrameworkPlugin::get_instance()` with `ReduxFrameworkPlugin::instance()` since `get_instance` did not exist.  Replaced `plugin_meta_demo_mode_link` with `plugin_metalinks` since `plugin_meta_demo_mode_link` was renamed at some point.  Also, changed hook into remove_demo from `redux/plugin/hook` to `redux/loaded` since the former wasn't working.  It was being called way before any theme would load.