Council may remove the Mayor from office for gross misconduct, or malfeasance, misfeasance or nonfeasance in or disqualification for office, or for conviction while in office for a crime involving moral turpitude, or if adjudicated legally incompetent, or for a violation of his oath of office, or persistent failure to perform the duties of his office; provided, however, that such removal shall not take place without the affirmative vote of two-thirds of the entire Council; nor until the accused Mayor shall have been notified in writing of the charge or charges against him at least fifteen (15) days in advance of the public hearing upon such charge or charges; and he or his counsel shall have been given an opportunity to be heard, present evidence, and examine, under oath, all witnesses appearing in support of such charge or charges.