Remove remaining checks for rbt64

These checks have been redundant since the `rbtdb64` implementation
was removed in 2018 (commit 784087390a). It isn't possible to create
a zone that uses `database "rbt64"` now that the `rbt64` database
implementation has been removed, so the checks will always fail.

lib/bind9/check.c

@@ -3738,7 +3738,7 @@ check_zoneconf(const cfg_obj_t *zconfig, const cfg_obj_t *voptions,
 	}
 
 	/*
-	 * If the zone type is rbt/rbt64 then primary/hint zones require file
+	 * If the zone type is rbt then primary/hint zones require file
 	 * clauses. If inline-signing is used, then secondary zones require a
 	 * file clause as well.
 	 */
@@ -3759,8 +3759,7 @@ check_zoneconf(const cfg_obj_t *zconfig, const cfg_obj_t *voptions,
 		result = ISC_R_FAILURE;
 	} else if (!dlz && (tresult == ISC_R_NOTFOUND ||
 			    (tresult == ISC_R_SUCCESS &&
-			     (strcmp("rbt", cfg_obj_asstring(obj)) == 0 ||
-			      strcmp("rbt64", cfg_obj_asstring(obj)) == 0))))
+			     strcmp("rbt", cfg_obj_asstring(obj)) == 0)))
 	{
 		isc_result_t res1;
 		const cfg_obj_t *fileobj = NULL;

lib/dns/rbtdb.h

@@ -34,8 +34,8 @@ dns_rbtdb_create(isc_mem_t *mctx, const dns_name_t *base, dns_dbtype_t type,
 		 void *driverarg, dns_db_t **dbp);
 
 /*%<
- * Create a new database of type "rbt" (or "rbt64").  Called via
- * dns_db_create(); see documentation for that function for more details.
+ * Create a new database of type "rbt". Called via dns_db_create();
+ * see documentation for that function for more details.
  *
  * If argv[0] is set, it points to a valid memory context to be used for
  * allocation of heap memory.  Generally this is used for cache databases

lib/dns/zone.c

@@ -1924,9 +1924,7 @@ dns_zone_rpz_enable(dns_zone_t *zone, dns_rpz_zones_t *rpzs,
 	 * Only zones that are loaded instead of mmap()ed create the
 	 * summary data and so can be policy zones.
 	 */
-	if (strcmp(zone->db_argv[0], "rbt") != 0 &&
-	    strcmp(zone->db_argv[0], "rbt64") != 0)
-	{
+	if (strcmp(zone->db_argv[0], "rbt") != 0) {
 		return (ISC_R_NOTIMPLEMENTED);
 	}
 
@@ -2163,8 +2161,7 @@ zone_load(dns_zone_t *zone, unsigned int flags, bool locked) {
 
 	INSIST(zone->db_argc >= 1);
 
-	rbt = strcmp(zone->db_argv[0], "rbt") == 0 ||
-	      strcmp(zone->db_argv[0], "rbt64") == 0;
+	rbt = strcmp(zone->db_argv[0], "rbt") == 0;
 
 	if (zone->db != NULL && zone->masterfile == NULL && rbt) {
 		/*