Clean up hook_cron implementation.

Problem/Motivation

Follow up to https://www.drupal.org/i/1662752 →

The latest commit adds some great functionality but has a stray devel-related call (dpq) and is missing a docblock for hook_cron. I'll submit a MR to clean that up.

See maillog_cron():

• https://git.drupalcode.org/project/maillog/-/commit/137be4329bac431ab5b2...