code: public function dropIndex($name, $table)
      {
          /** @var Schema $schema */
          $schema = $this->db->getSchema();
          foreach ($schema->getTableUniques($table) as $unique) {
              if ($unique->name === $name) {
                  return $this->dropUnique($name, $table);
              }
          }
  
          return 'DROP INDEX ' . $this->db->quoteTableName($name) . ' ON ' . $this->db->quoteTableName($table);
      }
docs: {@inheritdoc}
  @see http://www.cubrid.org/manual/93/en/sql/schema/table.html#drop-index-clause