Remove fvimacnvALT from iset.mm
Remove fvimacnvALT from iset.mm

Although this proof looks like it could be intuitionized just fine
(similarly to the other fvex to funfvex intuitionizations),
we already have one proof of fvimacnv so might as well leave the
other one for set.mm only.

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