net: silence another epoll print

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/5752048

src/pkg/net/fd_linux.go

@@ -84,7 +84,8 @@ func (p *pollster) StopWaiting(fd int, bits uint) {
 
 	events, already := p.events[fd]
 	if !already {
-		print("Epoll unexpected fd=", fd, "\n")
+		// The fd returned by the kernel may have been
+		// cancelled already; return silently.
 		return
 	}