Remove poormanscron mention from the project page

Problem/Motivation

We advise against using Poormanscron. Use cron(tab) and cron.php instead. We use locking which should prevent sending multiple copies. However you can expect much more timing accuracy and reproducibility when using a real cron. If you want support, please setup cron first

poormanscron hasn't existed since Drupal 6, this could use an update.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes