synchronized void decrementDeletedReplica(String blobId, String host, int port) {
    objectTracker.get(blobId).decrementDeleted(host, port);
  }
Nullifies the deletion notification for {@code blobId} on {@code host}:{@code port}.
This method should not be used concurrently with the await functions
@param blobId the blob ID whose deletion notification needs to be nullified
@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/MockNotificationSystem.java