I've seen professional installers remove the old glass by cutting the butyl with a special tool that looks like a saw-zall with a 6-8" long by 2-3" wide beavertail shaped blade with saw teeth on the end. The teeth were flat and not offset. The recipricating action of the tool cut the butyl effectively w/o damaging the glass.

You may be able to get a blade for a saw-zall if you have one or just try to make something with 14 gauge steel and a grinder, etc. See if the glass installers have a tool you can use or pay them to do it as previously suggested.