Delete unused taxonomy terms

delete_taxonomy_term

<?php

$field = 'VOCABULARY_MACHINE_NAME';

// Search for unused taxonomy terms.
$query = db_query('SELECT DISTINCT field_' . $field .  '_tid FROM field_data_field_' . $field . ' WHERE field_' . $field . '_tid NOT IN (SELECT tid FROM taxonomy_term_data)');
$results = $query->fetchCol();
 
// Output results with devel.
dpm($results);

// Delete unused taxonomy terms.
foreach ($results as $result) {
  db_delete('field_data_field_' . $field)
    ->condition('field_' . $field . '_tid', $result)
    ->execute();
}