This happens with many items, and it's really rediculous. The fact that quest items can't be dropped whilst you have that certain quest uncompleted is fair enough, but when you complete the quest and you still can't drop it makes me wonder why Bethesda haven't done something about it.
However, I know how to get rid of The Elder Scroll. Volsung said the Thieves Guild option, but as that's not applicable to you, if you travel to the College of Winterhold and go into The Arcanaeum, speak to the Orc who basically owns the library and there should be an option to give it him, in exchange for 2000 gold too.