Does anybody know where to find Proventus Avencci? He was replaced with Brill after the raid on Whiterun. Does anybody know where to find him, or how to delete the quest?

He should have moved into Solitude at the Blue Palace if you haven't completed all of the Civil War yet and if you have then why didn't you already take the sword to him as he's about a 2 minute walk from where you get the request... You could also try turning on the objectives marker for Miscellaneous Objectives and turn that speciffic one on and see where he is... Otherwise it seems that you've pretty well screwed yourself on that objective...

Ok, thanks for the tip. I got the quest after we already attacked Whiterun. So, the blue palace it is