Most bosses are already taken down that quick by the people that have the Gjallarhorn. And so many people already have it, why wouldn't he sell it again? Now, I wouldn't be surprised if Bungie lowered the chances of him selling it, but I highly doubt they removed it completely from his inventory, mainly because he already has sold it. They already knew long before they released the game what a beast they had created, so why would they have Xûr sell it once if they were just going to remove it from his inventory later? Just doesn't make sense.