Fix markup in iset.mm
Fix markup in iset.mm

These are forward references in the section on indexed union and
intersection. At df-iun , remove references to fniunfv and
funiunfv . At viin , remove proofs in words which may or may
not hold (and one of which has a forward references to intv ).

metamath/set.mm,metamath/set.mm,metamath/set.mm