net: remove dead code in Write

Also, please be informed that the Write method on both connected and
unconnected-mode sockets may return a positive number of bytes written
with timeout or use of closed network connection error.

Change-Id: I2e2e6192e29cef4e9389eb0422c605c6d12e6a3c
Reviewed-on: https://go-review.googlesource.com/9466Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/fd_unix.go

@@ -311,7 +311,6 @@ func (fd *netFD) Write(p []byte) (nn int, err error) {
 			}
 		}
 		if err != nil {
-			n = 0
 			break
 		}
 		if n == 0 {