Currently it isn't possible to set the forks paramter to an empty value.
If you want a consistent forks value for your playbooks runs, you set the forks value in a ansible.cfg file.
This value isn't used for playbook runs by the ansible plugin, because it always adds the '-f' parameter with the forks value to the command.
It would be great if you would change the behaviour so that the '-f' option isn't passed to the ansible command if forks is empty.
This would give you playbook runs with a consistent forks value if you run the playbook on your workstation (wich respects ansible.cfg forks values) and if you run you playbook with jenkins and the ansible plugin (which currently not respect the ansible.cfg forks value).