There's no way to discard unwanted items which have no price. It's a matter of game design. If you can get rid of everything then you might discard something important by mistake. If something has a price then it can be sold and then you might sell something you need later.So, ideally, every quest item should have a sale value of zero GP and the game should take it from you when it is no longer needed. This is done in a few places but not all. The designers didn't have time to tidy this up; to do so would have required very careful scripting in some places. (You might be aware of how they messed up the removal of the Orbs of Linking.)
Instead, they just made sure that the player would have so much inventory space that carrying extra items wouldn't be a problem -- except that going though such large inventories is itself a problem.
