Ruler visibility is not really a session-prop.

This fixes an issue with the session being marked as dirty without
any actual change. Also this method indirectly called during session-load
when the UI state is restored, a potential race with "Session::Loading"
being unset.

gtk2_ardour/editor_rulers.cc

@@ -307,7 +307,6 @@ Editor::store_ruler_visibility ()
 	node->set_property (X_("videotl"), ruler_video_action->get_active());
 
 	_session->add_extra_xml (*node);
-	_session->set_dirty ();
 }
 
 void