Whenever any bonds issued under the provisions of this chapter remain unsold for the period of six months after having been offered for sale in the manner prescribed by the board of supervisors, the board of trustees of the library district for or on account of which the bonds were issued, or of any library district composed wholly or partly of territory which, at the time of holding the election authorizing the issuance of the bonds, was embraced within the district for or on account of which the bonds were issued, may petition the board of supervisors to cause the unsold bonds to be withdrawn from market and canceled.