Remove RelationSetIndexList().

In the wake of commit f912d7dec, RelationSetIndexList isn't used any
more.  It was always a horrid wart, so getting rid of it is very nice.
We can also convert rd_indexvalid back to a plain boolean.

Discussion: https://postgr.es/m/28926.1556664156@sss.pgh.pa.us

src/include/utils/rel.h

@@ -60,8 +60,8 @@ typedef struct RelationData
 	bool		rd_islocaltemp; /* rel is a temp rel of this session */
 	bool		rd_isnailed;	/* rel is nailed in cache */
 	bool		rd_isvalid;		/* relcache entry is valid */
-	char		rd_indexvalid;	/* state of rd_indexlist: 0 = not valid, 1 =
-								 * valid, 2 = temporarily forced */
+	bool		rd_indexvalid;	/* is rd_indexlist valid? (also rd_pkindex and
+								 * rd_replidindex) */
 	bool		rd_statvalid;	/* is rd_statlist valid? */
 
 	/*

src/include/utils/relcache.h

@@ -66,9 +66,6 @@ extern void RelationGetExclusionInfo(Relation indexRelation,
 						 Oid **procs,
 						 uint16 **strategies);
 
-extern void RelationSetIndexList(Relation relation,
-					 List *indexIds);
-
 extern void RelationInitIndexAccessInfo(Relation relation);
 
 /* caller must include pg_publication.h */