All amounts collected and held in a sinking fund pursuant to Article IX, Section V, Paragraph VI of the Constitution of Georgia, irrevocably pledged and appropriated to the retirement of the outstanding bonded indebtedness for which they are collected, which can be legally used for no other purpose, shall be deductible from total bonded debt in computing the constitutional limitation on the bonded indebtedness of a political subdivision of this state under Article IX, Section VII of the Constitution of Georgia.