At the time and place designated in the notice for hearing the petition, or at any subsequent time to which the hearing is postponed, the supervising board of supervisors shall hear any reasons that are submitted for or against the granting of the petition, and if they deem it for the best interests of the library district named in the petition that the unsold bonds be canceled, they shall make and enter an order in the minutes of their proceedings that the unsold bonds be canceled. Thereupon the bonds, and the vote by which they were authorized to be issued, shall cease to be of any validity whatever.