[ticket/12576] Remove cron from common.php

PHPBB3-12576
[ticket/12576] Retrieve the cron_manager in cron.php if needed

PHPBB3-12576
Merge pull request #2484 from Nicofuma/ticket/12576

[ticket/12576] Remove cron from common.php

* Nicofuma/ticket/12576:
  [ticket/12576] Retrieve the cron_manager in cron.php if needed
  [ticket/12576] Remove cron from common.php