remove useless flush method

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)

engine/streams.go

@@ -109,18 +109,6 @@ func (o *Output) Write(p []byte) (n int, err error) {
 	return len(p), firstErr
 }
 
-func (o *Output) Flush() {
-	o.Mutex.Lock()
-	defer o.Mutex.Unlock()
-	for _, dst := range o.dests {
-		if f, ok := dst.(interface {
-			Flush()
-		}); ok {
-			f.Flush()
-		}
-	}
-}
-
 // Close unregisters all destinations and waits for all background
 // AddTail and AddString tasks to complete.
 // The Close method of each destination is called if it exists.