[CRM] Clean Business Model

- Task: https://www.odoo.com/web#id=8862&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.bMVjkCKLyLgbuEzl
As per specification we had to remove default_section_id and related code from user, i think in that context this method also removed but it's wrong so i added it back.