[REM] project : Remove reviewer field

- Task: https://www.odoo.com/web?debug=#id=9647&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.CfGLHrex0PwiSFux
Actually the whole method is not necessary anymore I think, you can go back to the default version of mail.thread.