Remove the usage of lastStatementDate in account KVP

Summary:
Very old versions of KMyMoney used to store the reconciliation date in
the KVP as "lastStatementDate". Since we don't use it anymore, we get
rid of the references.

Fixes T9254

Test Plan: Ran adjusted testcases

Reviewers: wojnilowicz

Reviewed By: wojnilowicz

Maniphest Tasks: T9254

Differential Revision: https://phabricator.kde.org/D14441