Fix crash when group becomes empty but has a subgroup

Session::route_removed_from_route_group() did not delete
the group when it had a subgroup associated with it.
This way it was possible to have an empty track-group that
resulted in various edge-case crashes/assert().

libs/ardour/route_group.cc

@@ -283,6 +283,7 @@ RouteGroup::remove (std::shared_ptr<Route> r)
 		}
 		routes->erase (i);
 		if (routes->empty()) {
+			_subgroup_bus.reset ();
 			group_master.reset ();
 			_group_master_number = -1;
 		}