Just Change the Description explaining it's no longer being supported and remove the files. and ask a dev staff to remove it maybe?
go to bukkit's home, look to the left side. You should find "Online Bukkit Dev Staff" click one and click "Start a Conversation"
While on bukkitdev, go to the top right corner that has how many unread pm's you have. Click on that, and then towards the right side of the screen and up a bit is the New Message button. Simply send a message to a bukkitdev staff member. A good one might be @h31ix as he's on the bukkit forums right now. EDIT: @Kodfod 's method is better.
You can either PM a staff on the forums or on BukkitDev, please include a link to the project you want deleted.