Executes the DELETE query.
The return value is dependent on the database connection.
Overrides Query::execute
public function execute() {
  $values = array();
  if (count($this->condition)) {
    $this->condition
      ->compile($this->connection, $this);
    $values = $this->condition
      ->arguments();
  }
  return $this->connection
    ->query((string) $this, $values, $this->queryOptions);
}