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

src/backend/utils/activity/pgstat.c

@@ -307,7 +307,6 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.fixed_amount = false,
 
 		.accessed_across_databases = true,
-		.named_on_disk = true,
 
 		.shared_size = sizeof(PgStatShared_ReplSlot),
 		.shared_data_off = offsetof(PgStatShared_ReplSlot, stats),

src/include/utils/pgstat_internal.h

@@ -193,12 +193,6 @@ typedef struct PgStat_KindInfo
 	 */
 	bool		accessed_across_databases:1;
 
-	/*
-	 * For variable-numbered stats: Identified on-disk using a name, rather
-	 * than PgStat_HashKey. Probably only needed for replication slot stats.
-	 */
-	bool		named_on_disk:1;
-
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
 	 * PgStatShared_HashEntry->body).
@@ -239,7 +233,7 @@ typedef struct PgStat_KindInfo
 	void		(*reset_timestamp_cb) (PgStatShared_Common *header, TimestampTz ts);
 
 	/*
-	 * For variable-numbered stats with named_on_disk. Optional.
+	 * For variable-numbered stats. Optional.
 	 */
 	void		(*to_serialized_name) (const PgStat_HashKey *key,
 									   const PgStatShared_Common *header, NameData *name);