un-bound dremora isn't despawning after I complete the conjuration ritual spell quest
This is a known issue across all three platforms that Skyrim is available on. Fortunately it doesn't have any real impact on your game and you can safely ignore it.
Both the Wikia and UESP articles list this bug. It is also supposedly resolved (or at least occurs less frequently) with version 1.2.4 of the Unofficial Skyrim Patch.