Remove the edm4hep::ObjectID from the datamodel

[tmadlener]

Currently edm4hep defines its own ObjectID:

EDM4hep/edm4hep.yaml

Lines 110 to 120 in 66c90fc

	#------ ObjectID helper struct for references/relations
	edm4hep::ObjectID:
	Members:
	- int32_t index
	- int32_t collectionID
	ExtraCode:
	includes: "#include <podio/ObjectID.h>\n"
	declaration: "
	ObjectID() = default;\n
	ObjectID(const podio::ObjectID& id ): index(id.index), collectionID(id.collectionID) {}\n
	"

This is slightly different from the podio one which basically boils down to

struct ObjectID {
  int index;
  uint32_t collectionID;
};

As far as I can tell this is actually unused and we should be able to achieve the same (intended) functionality.