unix: remove outdated comment

The comment removed by this commit is no longer relevant, as
the function's return type changed from int to void.

PR-URL: https://github.com/libuv/libuv/pull/804
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/pipe.c

@@ -200,9 +200,6 @@ out:
   if (err)
     uv__io_feed(handle->loop, &handle->io_watcher);
 
-  /* Mimic the Windows pipe implementation, always
-   * return 0 and let the callback handle errors.
-   */
 }