scope: on unified, make sure to unwatch all PIDs once they've been moved to the cgroup scope
Commit 428a9f6f freed u->pids which is
problematic since the references to this unit in m->watch_pids were no more
removed when the unit was freed.
This patch makes sure to clean all this refs up before freeing u->pids by
calling unit_unwatch_all_pids().
(cherry picked from commit e9eec8b5)