chore: Drop solver_fee column from order_execution table

[sunce86]

Background

This field was added when surplus_fee for limit orders was calculated before order execution (and even was periodically recalculated before, during and after the limit order was executed). The field was added to capture the value at execution time.

Since this functionality is dropped and limit orders now get the fee from solvers, we can also drop this column, since for limit orders is zero, while for market orders is equal to solvers_fee from the order struct which is constant over time, so it's basically a duplicated data.

[sunce86]

Closed with #2359