code: public static EntityUnlinkOperation unlinkContentKey(String assetId,
              String contentKeyId) {
          return new EntityUnlinkOperation(ENTITY_SET, assetId, "ContentKeys", contentKeyId);
      }
docs: unlink a content key.
  
  @param assetId
  the asset id
  @param contentKeyId
  the content key id
  @return the entity action operation