Discontinuance of use of non-conforming building or structure. A building, structure or portion thereof, all or substantially all of which is designed or intended for a use which is not permitted in the district in which it is located and which is vacant on the effective date of this Chapter or thereafter becomes vacant and remains unoccupied or is not used for a continuous period of ninety (90) days, shall not thereafter be occupied or used, except by a use which conforms to the use regulations of the district in which it is located. A building or structure which is non-conforming only as to height, yard, parking or loading requirements shall not be subject to this provision.