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.
On Oracle, `rake db:migrate` seems to complete (no error messages), but it doesn't seem to drop the column either. On `rake db:rollback` it complains: "column being added already exists in table". Maybe the account I'm using isn't allowed to drop columns. But if so, why no errors with the initial `migrate`?