Remove item_info

Remove item_info

This type was just a typedef to item_def. It was added in 8f49829c0752,
apparently with the idea that item_info would be used for item_defs that
represented id info, and item_def otherwise. However, this convention
wasn't even documented, let alone enforced by an actual type distinction
(and c++ never added a strict typedef), so it has undergone drift and
largely now exists to confuse people. I could imagine redoing this so
that the type checker will do something, but the id system is also much
less important now than it was at the time of the original commit, so it
seems cleaner just to simplify.