fix a leftover use of shared_from_this()

This was left from the experimental phase of shared route groups,
when the GUI was still using raw pointers.

gtk2_ardour/editor_route_groups.cc

@@ -207,7 +207,7 @@ EditorRouteGroups::remove_selected ()
 		std::shared_ptr<RouteGroup> rg = (*iter)[_columns.routegroup];
 
 		if (rg) {
-			_session->remove_route_group (rg->shared_from_this());
+			_session->remove_route_group (rg);
 		}
 	}
 }