threaded-resolver: revert Curl_expire_latest() switch

The switch to using Curl_expire_latest() in commit cacdc27f52b 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