Remove PgStat_KindInfo.named_on_disk

Remove PgStat_KindInfo.named_on_disk

This field is used to track if a stats kind can use a custom format
representation on disk when reading or writing its stats case.  On HEAD,
this exists for replication slots stats, that need a mapping between an
internal index ID and the slot names.

named_on_disk is currently used nowhere and the callbacks
to_serialized_name and from_serialized_name are in charge of checking if
the serialization of the stats data should apply, so let's remove it.

Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/ZmKVlSX_T5YvIOsd@paquier.xyz