Remove unused classical theorems from iset.mm.
Remove unused classical theorems from iset.mm.

Specifically mt3i, pm2.61ddan, pm2.61dda, pm2.61nii, and pm2.61iii.
They don't do a better job of contrasting with intuitionistic logic
than, say, pm2.61 itself.

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