Fix bug where payee default category disappears on update.

After setting a default category on a payee, and then pressing update, the default category would mysteriously disappear. Changing focus to another payee and then returning would cause it to reappear. The fix was that calling setCurrentIndex(-1) was circumventing the API of KMyMoneyAccountCombo. Instead, clearSelection() should have been called. The former call cleared the widget, but failed to clear m_lastSelectedAccount, which prevented the default category from being set back to the same value two lines later.