Im pretty sure its the old-type glue they used on these vehicles. The glue is what yellows/browns. I know because my car was in a half-bumper crush, and the passanger side has new headlights... no browning. I am planning on outlining my headlights in black, to get rid of the nastyness, at least I may do it.
Basically, you cannot get rid of it, unless you break all the glue that is holding the front of the headlight to the reflector housing, scrape off all the glue, and replace with better glue.
Or you can buy new headlights for 75$ each.