Remove voluntary_contribution references

CHANGELOG:

### Fixed
- [#430](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/430) Remove voluntary_contribution references.

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voluntary_contribution field moved from easy_my_coop_es to La Feixa module:

https://github.com/coopdevs/vertical-cooperative/commit/528f3ea29575d72c322587c66a948e470d8aae1f