void decrementCount(String host, int port) {
      if (hosts.remove(getKey(host, port)) != null) {
        long finalCount = latch.getCount() + 1;
        if (finalCount > numberOfReplicas) {
          throw new IllegalArgumentException("Cannot add more replicas than the max possible replicas");
        }
        latch = new CountDownLatch((int) finalCount);
      }
    }
Nullifies the notification received (if any) on the given {@code host}:{@code port}.
This method is NOT thread safe and should not be used concurrently with other methods in this class like
await() and track().
@param host the host that to decrement on
@param port the port of the host that describes the instance along with {@code host}.
ambry-test-utils/src/main/java/com/github/ambry/server/MockCluster.java