Simplify one use of ScanKey in pg_subscription.c

The section of the code in charge of returning all the relations
associated to a subscription only need one ScanKey, but allocated two of
them.  This code was introduced as a copy-paste from a different area on
the same file by 7c4f5240, making the result confusing to follow.

Author: Peter Smith
Reviewed-by: Tom Lane, Julien Rouhaud, Bharath Rupireddy
Discussion: https://postgr.es/m/CAHut+PsLKe+rN3FjchoJsd76rx2aMsFTB7CTFxRgUP05p=kcpQ@mail.gmail.com

src/backend/catalog/pg_subscription.c

@@ -461,19 +461,18 @@ GetSubscriptionRelations(Oid subid)
 	List	   *res = NIL;
 	Relation	rel;
 	HeapTuple	tup;
-	int			nkeys = 0;
-	ScanKeyData skey[2];
+	ScanKeyData skey[1];
 	SysScanDesc scan;
 
 	rel = table_open(SubscriptionRelRelationId, AccessShareLock);
 
-	ScanKeyInit(&skey[nkeys++],
+	ScanKeyInit(&skey[0],
 				Anum_pg_subscription_rel_srsubid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(subid));
 
 	scan = systable_beginscan(rel, InvalidOid, false,
-							  NULL, nkeys, skey);
+							  NULL, 1, skey);
 
 	while (HeapTupleIsValid(tup = systable_getnext(scan)))
 	{