Invoice Line price unit set to amount untaxed

CHANGELOG:

### CHANGED
- [#693](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/693) Invoice Line price unit set to amount untaxed

Related card: https://trello.com/c/Q10vDVwS/1070-preu-unitari-a-linies-de-factura-que-venen-dopencell

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