Remove validation of previous_service in BroadbandISPInfo

CHANGELOG:

### Removed
- [#648](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/648) Remove validation of previous_service in BroadbandISPInfo

Related card: https://trello.com/c/KTkzOlwv/1005-hacer-no-requerido-el-previousservice-en-odoo

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