remove support for json object type monitems
Fix embarrassing compile error, don't look.
Add death_drops member to class mtype.

Contains the ident of the item group used to create items drooped upon death.
It is loaded from json as part of loading the monster type.
Json can contain either a group name or an embedded item group object.

And add check for death_drops item group to MonsterGenerator::check_monster_definitions
move death drops from monitem entries into monster definition
remove item_chance from monster definition, not used anymore.

It still remains in json for some monsters so the value can be included into the death drops item group in the future.