Use proper placeholder arrays for IN clause queries

This applies to both Drupal 6 and Drupal 7 versions of Session API although the syntax is different since db_placeholders() was deprecated in Drupal 7.

This query call in session_api_cron() should use proper placeholder logic

See:
(Drupal 6) http://api.drupal.org/api/drupal/includes--database.inc/function/db_plac...
(Drupal 7) http://drupal.org/node/310072

db_query('DELETE FROM {session_api} WHERE sid IN (' . implode(',', $outdated_sids) . ')');