(PECL solr >= 0.9.2)
SolrDocument::deleteField — Removes a field from the document
public SolrDocument::deleteField ( string $fieldName ) : bool
Removes a field from the document.
fieldName
Name of the field
 Returns true on success or false on failure. 
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    https://www.php.net/manual/en/solrdocument.deletefield.php