Remove default start date from change tariff wizard
CHANGELOG:
### Changed
- [#848](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/848) Remove default start date from change tariff wizard
Related card: https://trello.com/c/UJk25m4d/1331-treure-data-per-defecte-del-wizard-de-canvi-de-tarifa