NPCs: uncache weapon value on weapon removal

Fixes #32613

The trade interface allowed you to remove an NPC's weapon without
removing the cached value of the weapon.

Invalided the weapon value cache entry when a weapon is removed.

src/character.cpp

@@ -1251,6 +1251,7 @@ item Character::remove_weapon()
 {
     item tmp = weapon;
     weapon = item();
+    cached_info.erase( "weapon_value" );
     return tmp;
 }