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

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