Remove documents-to-elements helpers

[limenet]

E.g. \Valantic\ElasticaBridgeBundle\Index\AbstractIndex::getDocumentFromElement, \Valantic\ElasticaBridgeBundle\Index\AbstractIndex::searchForElements, and \Valantic\ElasticaBridgeBundle\Index\AbstractIndex::documentResultToElements as they encourage loading data from Elasticsearch and Pimcore which is often inefficient.