Can anyone tell me how to remove those useless keys after the quests?

What you mean Nameless Crazy? Can you not drop the keys on the ground or store them in some poor fellas bookcase?

just drop them.

If you later find out they have something to do with the plot you can find(buy) them in the temple again (in the divining pool)

i have had it before where the key or some other items dont wanna move from their slot they are in but you can right click on that item then choose the drop selection and that should drop it.

if you keep them in your inventory, they'll disappear once you enter a new chapter.