Remove cancel button from planned activities

CHANGELOG:

### Changed
- [#729](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/729) Remove cancel button from planned activities

Related card: https://trello.com/c/dirI9oWY/1131-eliminar-opci%C3%B3-cancellar-de-les-activitats-no-finalitzades-a-la-vista-del-partner

---