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.
There sure is a lot of setup for this tiny test. We should probably at least clean up the formatting in the `before`, even if we don't actually clean up the tests. Should we include a test that checks `#amount`?