It's now in the semaphore
table with the key name "cron". This can be removed via a drush sqlq "DELETE FROM semaphore WHERE name = 'cron';"
command.
drush sqlq "DELETE FROM semaphore WHERE name = 'cron';"
Or, if you can execute PHP in Drupal bootstrapped environment, you can run the line:
\Drupal::lock()->release('cron');
\Drupal::lock()->release('cron');