Your skis won't last very long without topsheets. Also they'll ski differently.
But If you really want to then I don't think a heat gun would work since it's probably epoxy. You could sand them off, but that would take a while unless you have a nice big belt sander. You might have luck with a chisel and hammer, but unless you've got some good chisel skills you'll probably damage the core a bunch.
If I was tasked with doing a good removal job of topsheets this is what I'd do:
-measure the thickness of the topsheet
-set router to that thickness
-make cuts across the ski at maybe 1-2 inch intervals
-use a chisel to pry off each of the little sections
-palm sander to smooth it out