[FIX] point_of_sale,pos_sale: ignore fixed tax in downpayment

[robinengels]

When you made a downpayment on an order that contained product with fixed amount taxes, the tax would be paid multiple times.

Steps to reproduce:

• Create a tax T1 with a fixed amount of 10€
• Create a product P1 using the tax T1
• Make a sale order and add the product P1 to it
• Open PoS and make a downpayment for the sale order (e.g 50%)
• You will already pay the 10€ of tax
• Now if you make a second downpayment (e.g. 10%)

Observation: You still have the 10€ tax to pay

Why the fix:

We match the behavior of sales app, and ignore the fixed price taxes when creating the downpayment lines.

opw-4163579

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[robodoo]