account_payment_sale: remove unneeded code

Remove inherit of sale.advance.payment.inv: no need to inherit
_prepare_invoice_values() because it calls _prepare_invoice() from the
sale.order object

account_payment_sale/__init__.py

@@ -1,2 +1 @@
 from . import models
-from . import wizard

account_payment_sale/wizard/__init__.py

@@ -1 +0,0 @@
-from . import sale_make_invoice_advance

account_payment_sale/wizard/sale_make_invoice_advance.py

@@ -1,14 +0,0 @@
-# Copyright 2016-2020 Akretion - Alexis de Lattre
-# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
-
-from odoo import models
-
-
-class SaleAdvancePaymentInv(models.TransientModel):
-    _inherit = "sale.advance.payment.inv"
-
-    def _prepare_invoice_values(self, order, so_line):
-        """Copy payment mode from sale order to invoice"""
-        vals = super()._prepare_invoice_values(order, so_line)
-        order._get_payment_mode_vals(vals)
-        return vals