By the adoption of an ordinance by a four-fifths (4/5) vote, the board of supervisors may abolish the system theretofore established in the county pursuant to this chapter or its predecessor. The abolishment of the system shall not impair the right of any person to whom pension rights have accrued to continue to receive such pension after the abolishment of the system, and if the fund is inadequate to pay such pensions, they shall be paid from and constitute a charge upon the general fund of the county.