ObserverZooKeeperServer.shutdown() is redundant

After the FIX of ZOOKEEPER-1796, LearnerZooKeeperServer.shutdown() should be responsible for the shutdown logic of both the follower and observer. ObserverZooKeeperServer.shutdown() seems redundant, because it is not in the call stack of Observer.shutdown(). (Note that FollowerZooKeeperServer does not have the shutdown() method.)

Related analysis can be found in ZOOKEEPER-4712