there is no quest. the black-briars, thieves guild members, and jarl are essential and will never, ever ever be stopped because their petty criminal racket is somehow above the abilities of dovahkiin the GOD SLAYER.
but really, before we do any of that i think the people of riften really need to shift their priorities to the atrocious state of the waste removal services. the corpses of thieves keep piling up, but nobody ever thinks to get rid of them!