[IMP] l10n_ar_tax: revert withholding rounding patch and improve calculation

Revertimos el cambio de este pull request #1218 y mejoramos cálculo de retención tal como lo hizo odoo aquí odoo/odoo@ac521b2#diff-dab181eb3722c2603ab3586daa152293cfa7b98bdccc7bd7508eec8062de67d9R71
Ticket: 101778

Author: pablohmontenegro

l10n_ar_tax/models/l10n_ar_payment_withholding.py

@@ -103,11 +103,9 @@ def _tax_compute_all_helper(self):
             product=False,
             partner=False,
             is_refund=False,
+            rounding_method="round_per_line",
         )
-        tax_amount = self.currency_id.round(taxes_res["total_included"] - taxes_res["total_excluded"])
-        # TODO: When Odoo fixes the compute_all method of account_tax, uncomment the line below and
-        # remove the line above. See Adhoc ticket 101778 for more information.
-        # tax_amount = taxes_res["taxes"][0]["amount"]
+        tax_amount = taxes_res["taxes"][0]["amount"]
         tax_account_id = taxes_res["taxes"][0]["account_id"]
         tax_repartition_line_id = taxes_res["taxes"][0]["tax_repartition_line_id"]