On or before the third day after the last day for filing nominating petitions, each person for whom a valid, nominating petition has been filed, or someone acting on his behalf, shall file with the Clerk an affidavit that he is possessed of all of the qualifications set forth in this Charter for an elective city office. Such affidavit shall be on a form provided by the Clerk. If such affidavit be not filed as herein required, the candidate shall be deemed to have withdrawn and his name shall not be placed on any election ballot.