Remove synchronized block from MasterServiceStubMaker#releaseZooKeeperWatcher()

This is a follow up to HBASE-11460

elserj found that in 0.98, the synchronized block below should have been taken out (as was done for branch-1 +):

      synchronized (masterAndZKLock) {
        if (keepAliveZookeeperUserCount.decrementAndGet() <= 0 ){

keepAliveZookeeperUserCount is an AtomicInteger. There is no need for the synchronized block.

This issue is to remove the synchronized block.