chore: Completely remove executedFeeAmount from API

[sunce86]

Background

Once all executed fees are in the same token (surplus token), we can show following in the API:

1. executedFee = executedFee + executedFeeAmount
2. executedFeeAmount = 0 for ALL orders

as there are no clients that are interested in executedFeeAmount solely, and also, this field is only kept for legacy orders.

This effectively means we can remove executedFeeAmount from API.

Acceptance criteria

There is only ONE fee in the API that represents the total fee that was taken for execution of an order, and it's called executedFee.

[sunce86]

We should also consider completely removing executedSellAmountBeforeFees as I believe frontend is using only executedSellAmount for these legacy orders.