Drop example already covered in official docs

This entire example is available at:
- https://kubernetes.io/docs/tutorials/stateful-application/mysql-wordpress-persistent-volume/
- https://github.com/kubernetes/website/tree/main/content/en/docs/tutorials/stateful-application/

Signed-off-by: Maciej Szulik <soltysh@gmail.com>

Author: soltysh