Remove amount column from statement_rows [#86776]

The amount was only being set on StatementRow creation, which meant it missed
updates to OrderDetail#total. With this change, StatementRow#amount gets its
value directly from OrderDetail#total.
Can you run this over the NU branch over the full data there? If takes too long, you can probably make the loop StatementRow.joins(:order_detail).each and change the update to update_attribute. Not necessary if this is still reasonably fast (not sure how many StatementRows we have in the real database).