Resolve @todo in SearchApiSort::query()

I recently had the dubious pleasure of debugging Views exposed sorting and can now confirm that the code in \Drupal\search_api\Plugin\views\sort\SearchApiSort::query() is indeed required. Furthermore, we should let \Drupal\search_api\Plugin\views\query\SearchApiQuery::$orderby default to NULL, not [], to make the code work exactly as intended.
It also seems like we can remove the reference to the (ten years old) #2145547: Exposed sort order doesn't work when combined with non-exposed sort on the same field.