[FIX] l10n_br: wrong & pointless field redefinition
[FIX] l10n_br: wrong & pointless field redefinition

Fields `amount_type` on `account.tax` and `account.tax.template` are
already defined in the `account` module.
Redefining them here (with the same parameters) breaks every other
module that would have used `selection_add=` on those fields.

Actually, it is the case in `account_tax_python`, and thus, all the
localizations/customizations that depend on it were broken by this one.

(issue spotted by 11.0-nightly)

Closes #19812 #20596

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo