Remove useless .expire initialization from rdataslab

dns_rdataslab_fromrdataset() set .expire to rdataset->ttl, but the
only consumer (qpcache_addrdataset) immediately overwrote it with
now + rdataset->ttl.  Remove the redundant initialization and set the
expire time only once.

lib/dns/include/dns/rdataslab.h

@@ -191,10 +191,8 @@ dns_rdataslab_fromrdataset(dns_rdataset_t *rdataset, isc_mem_t *mctx,
  *
  * dns_rdataslab_fromrdataset() allocates space for a dns_slabheader object
  * and the memory needed for a raw slab, and partially initializes
- * it, setting the type, trust, and TTL fields to match rdataset->type,
- * rdataset->covers, rdataset->trust, and rdataset->ttl.  (Note that the
- * last field needs to be overridden when used in the cache database,
- * since cache headers use an expire time instead of a TTL.)
+ * it, setting the type, and trust fields to match rdataset->type,
+ * rdataset->covers, and rdataset->trust.
  *
  * Requires:
  *\li	'rdataset' is valid.

lib/dns/qpcache.c

@@ -2874,11 +2874,9 @@ qpcache_addrdataset(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
 	dns_slabheader_reset(newheader, node);
 
 	/*
-	 * By default, dns_rdataslab_fromrdataset() sets newheader->ttl
-	 * to the rdataset TTL. In the case of the cache, that's wrong;
-	 * we need it to be set to the expire time instead.
+	 * Set the correct expire time.
 	 */
-	setttl(newheader, rdataset->ttl + now);
+	setttl(newheader, now + rdataset->ttl);
 	if (rdataset->ttl == 0U) {
 		DNS_SLABHEADER_SETATTR(newheader, DNS_SLABHEADERATTR_ZEROTTL);
 	}

lib/dns/rdataslab.c

@@ -110,7 +110,6 @@ newslab(dns_rdataset_t *rdataset, isc_mem_t *mctx, isc_region_t *region,
 	*header = (dns_slabheader_t){
 		.headers_link = CDS_LIST_HEAD_INIT(header->headers_link),
 		.trust = rdataset->trust,
-		.expire = rdataset->ttl,
 		.dirtylink = ISC_LINK_INITIALIZER,
 		.nitems = nitems,
 	};