hiperfifo: remove unused variable

Closes https://github.com/curl/curl/pull/3919
diff --git a/docs/examples/hiperfifo.c b/docs/examples/hiperfifo.c
index c8e8bfd..d1228f8 100644
--- a/docs/examples/hiperfifo.c
+++ b/docs/examples/hiperfifo.c
@@ -146,7 +146,6 @@
 static int multi_timer_cb(CURLM *multi _Unused, long timeout_ms, GlobalInfo *g)
 {
   struct timeval timeout;
-  CURLMcode rc;
 
   timeout.tv_sec = timeout_ms/1000;
   timeout.tv_usec = (timeout_ms%1000)*1000;