unix: remove stale closing check in uv__write()

Introduced in 0db3274f but no longer necessary; uv__write() no longer
runs when the handle has been closed. Write callbacks receive a special
status code that informs them that the handle has been closed.

src/unix/stream.c

@@ -670,13 +670,6 @@ static void uv__write(uv_stream_t* stream) {
   int iovcnt;
   ssize_t n;
 
-  if (stream->flags & UV_CLOSING) {
-    /* Handle was closed this tick. We've received a stale
-     * 'is writable' callback from the event loop, ignore.
-     */
-    return;
-  }
-
 start:
 
   assert(uv__stream_fd(stream) >= 0);