When there remains in the possession of the board of directors after completion of its drainage system and payment of all demands against the district, any bonds authorized to be issued by the district but not sold and not necessary to be sold to raise funds for the use of the district, the board of directors may call a special election for the purpose of voting upon a proposal to destroy such unsold bonds, or so many of them as may be deemed advisable, or may submit the proposal at a general election.