How can a visual approach cancel an IFR flight plan when it is defined this way...?
FAA.gov:
VISUAL APPROACH- An approach conducted on an instrument flight rules (IFR) flight plan which authorizes the pilot to proceed visually and clear of clouds to the airport. (the rest is cut away to shorten the quote)
VISUAL APPROACH [ICAO]- An approach by an IFR flight when either part or all of an instrument approach procedure is not completed and the approach is executed in visual reference to terrain.