CLEANUP: resolvers: remove unused RSLV_UPD_OBSOLETE_IP flag

RSLV_UPD_OBSOLETE_IP was introduced with commit a8c6db8d2 ("MINOR: dns:
Cache previous DNS answers.") but the commit didn't make any use of it,
and today the flag is still unused. Since we have no valid use for it,
better remove it to prevent confusions.

include/haproxy/resolvers-t.h

@@ -277,7 +277,6 @@ enum {
 	RSLV_UPD_CNAME,            /* CNAME without any IP provided in the response */
 	RSLV_UPD_NAME_ERROR,       /* name in the response did not match the query */
 	RSLV_UPD_NO_IP_FOUND,      /* no IP could be found in the response */
-	RSLV_UPD_OBSOLETE_IP,      /* The server IP was obsolete, and no other IP was found */
 };
 
 struct proxy;