@Override
  public boolean unregisterFlushRequestListener(final FlushRequestListener listener) {
    return this.flushRequestListeners.remove(listener);
  }
Unregister the listener from MemstoreFlushListeners
@return true when passed listener is unregistered successfully.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreFlusher.java