It is probably one of two things: dry glazed with glazing tape (i.e. 3M bonding tape) or wet glazed with silicon caulk.

Acetone will dissolve the adhesive in the bonding tape and a sharp razor blade on edge will scrape off the silicon. Best to get it as clean as you can with a razor blade first.

Have you removed the glazing bead? If not...you will want one of these.

http://www.gordonglassusa.com/item/EZ1010/crl-ezd-glazer-deglazing-tool-EZ1010.htm?src=froogle

Here is a link that might be useful: De-Glazing tool