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
Edited by Pelayo García