I'm playing the New Game Plus mode, and I keep finding upgrade materials for weapons that I have already upgraded to level 5. Is there any other use for those materials? Is there any way to dispose of them, if they have no use?
There's no use for them, and there's no way to dispose of them. Fortunately, you really never have a reason to go into your backpack (unless you like flavor text), so the only reminder of the fact that you have way too many upgrade items is the counts you see beside each weapon when you visit the Forge.