Fix memory leak due to LogicalRepRelMapEntry.attrmap.

When rebuilding the relation mapping on subscribers, we were not releasing
the attribute mapping's memory which was no longer required.

The attribute mapping used in logical tuple conversion was refactored in
PG13 (by commit e1551f9) but we forgot to update the related code that
frees the attribute map.

Author: Hou Zhijie
Reviewed-by: Amit Langote, Amit Kapila, Shi yu
Backpatch-through: 10, where it was introduced
Discussion: https://postgr.es/m/OSZPR01MB6310F46CD425A967E4AEF736FDA49@OSZPR01MB6310.jpnprd01.prod.outlook.com

Author: Amit Kapila

src/backend/replication/logical/relation.c

@@ -313,6 +313,13 @@ logicalrep_rel_open(LogicalRepRelId remoteid, LOCKMODE lockmode)
 		int			i;
 		Bitmapset  *missingatts;
 
+		/* Release the no-longer-useful attrmap, if any. */
+		if (entry->attrmap)
+		{
+			pfree(entry->attrmap);
+			entry->attrmap = NULL;
+		}
+
 		/* Try to find and lock the relation by name. */
 		relid = RangeVarGetRelid(makeRangeVar(remoterel->nspname,
 											  remoterel->relname, -1),