[13.0] l10n_ar: afip_concept field not able to force value

[sgheller]

Impacted versions:

• 13.0

Steps to reproduce:

1. Create draft Invoice.
2. Try to change l10n_ar_afip_concept.

Current behavior:

• Field is readonly.

Expected behavior:

• Ability to change field value.

Field definition states the following in ´Help´:
help="A concept is suggested regarding the type of the products on the invoice **but it is allowed to force a different type if required.**"

If the readonly attribute is overriden and set to False, then the method _compute_l10n_ar_afip_concept(self) still changes the field value when posting the invoice.