code: public boolean removeReference(K key, ServiceReference<V> reference) {
          if (key == null || reference == null)
              return false;
          ConcurrentServiceReferenceElement<V> element = new ConcurrentServiceReferenceElement<V>(referenceName, reference);
          return elementMap.remove(key, element);
      }
docs: Removes the reference associated with the key.
  
  @param key Key associated with this reference
  @param reference ServiceReference associated with service to be unset.
  @return true if reference was unset (not previously replaced)