code: public boolean removeReference(K key, ServiceReference<V> reference) {
          if (key == null)
              return false;
  
          ConcurrentServiceReferenceSet<V> csrs = elementMap.get(key);
          if (csrs == null) {
              return false;
          }
          return csrs.removeReference(reference);
      }
docs: Removes the reference associated with the key.
  
  @param key Key associated with this reference
  @param reference ServiceReference for the target service
  @return true if reference was unset (not previously replaced)