Get fee_amount from trade (#2342)

# Description
Now that we [killed the
subsidies](cowprotocol/infrastructure#1031), the
`trade.fee_amount` is always equal to `order.metadata.solver_fee` so we
get this data directly from onchain data instead of querying the
database.

Not super important since we will drop the support for market orders in
the following period, but important for moving this part of the code
into `domain` and simplifying the `surplus` and `fee` calculation on the
future
[domain::settlement](https://github.com/cowprotocol/services/blob/domain-settlement/crates/autopilot/src/domain/settlement/mod.rs#L14-L18)
object.

## How to test
Existing tests.

crates/autopilot/src/database.rs

@@ -13,7 +13,6 @@ pub mod fee_policies;
 pub mod on_settlement_event_updater;
 pub mod onchain_order_events;
 pub mod order_events;
-pub mod orders;
 mod quotes;
 pub mod recent_settlements;