pkg/cors: remove http flush

The code is introduced in 7dce4c8fbb, and
the comments cannot explain the usefulness of the code at all.

pkg/cors/cors.go

@@ -73,10 +73,6 @@ func (h *CORSHandler) addHeader(w http.ResponseWriter, origin string) {
 // ServeHTTP adds the correct CORS headers based on the origin and returns immediately
 // with a 200 OK if the method is OPTIONS.
 func (h *CORSHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
-	// It is important to flush before leaving the goroutine.
-	// Or it may miss the latest info written.
-	defer w.(http.Flusher).Flush()
-
 	// Write CORS header.
 	if h.Info.OriginAllowed("*") {
 		h.addHeader(w, "*")