threaded-resolver: revert Curl_expire_latest() switch

The switch to using Curl_expire_latest() in commit cacdc27 was a
mistake and was against the advice even mentioned in that commit. The
comparison in asyn-thread.c:Curl_resolver_is_resolved() makes
Curl_expire() the suitable function to use.

Bug: http://curl.haxx.se/bug/view.cgi?id=1426
Reported-By: graysky

lib/asyn-thread.c

@@ -541,7 +541,7 @@ CURLcode Curl_resolver_is_resolved(struct connectdata *conn,
       td->poll_interval = 250;
 
     td->interval_end = elapsed + td->poll_interval;
-    Curl_expire_latest(conn->data, td->poll_interval);
+    Curl_expire(conn->data, td->poll_interval);
   }
 
   return CURLE_OK;