[project] Code cleaning & migration to new API

- Task: https://www.odoo.com/web/#id=8847&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.kZxs1A0VXfYsHCv2
I believe that can be written as `self.mapped('task_ids')`

/cc @rco-odoo