If, in any county of the first class having a charter form of government, any property located within the geographical boundaries of a county library district is now, or hereafter, included within the geographical boundaries of an urban public library district supported at least in part by taxation, the property now, or hereafter, included within the geographical boundaries of the urban public library district shall be excluded from the county library district, and the excluded property shall only be subject to taxation for library purposes by the urban public library district and shall no longer be subject to taxation for county library district purposes.