[FIX] 14.0 SO transaction amount: remove paid amount

[PierrickBrun]

While creating the transaction, shopinvader_payment uses the sale_order amount_total.

This works well when using 1 transaction. But when using several transactions, all of them are created with amount_total as amount.

It should be decremented, this is what this PR does

[codecov-commenter]

Codecov Report

Base: 87.12% // Head: 85.96% // Decreases project coverage by -1.15% ⚠️

Coverage data is based on head (ac4087e) compared to base (7059f93).
Patch coverage: 91.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             14.0      #79      +/-   ##
==========================================
- Coverage   87.12%   85.96%   -1.16%     
==========================================
  Files          38       50      +12     
  Lines         396      506     +110     
  Branches       42       60      +18     
==========================================
+ Hits          345      435      +90     
- Misses         38       57      +19     
- Partials       13       14       +1     

Impacted Files	Coverage Δ
shopinvader_payment/models/sale_order.py	93.33% <0.00%> (-6.67%)	⬇️
...pinvader_payment/services/abstract_payable_sale.py	100.00% <100.00%> (ø)
...nvader_payment/services/invader_payment_service.py	78.26% <0.00%> (-0.32%)	⬇️
shopinvader_payment/services/cart.py	100.00% <0.00%> (ø)
shopinvader_payment/services/__init__.py	100.00% <0.00%> (ø)
invader_payment/models/invader_payable.py	100.00% <0.00%> (ø)
shopinvader_payment/tests/test_payment.py	100.00% <0.00%> (ø)
invader_invoice_payment/models/__init__.py	100.00% <0.00%> (ø)
...er_payment_condition/models/shopinvader_payment.py	100.00% <0.00%> (ø)
... and 10 more

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[github-actions]

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