When a sanitary district has been discontinued by order of the board of supervisors, as provided in this subchapter, the order of the board of supervisors shall be filed in the office of the recorder in the county or counties in which the sanitary district is located. The agreement of the city in which the sanitary district is located and which has agreed to assume the duties, responsibilities and functions of the sanitary district shall also be filed along with, and as part of the order of the board of supervisors conveying and discontinuing the district.