When you are working with Drupal form API or in module development, most probably you met this problem to add a maanged file field(uplaoding a file via ajax inside the form API). Here I am showing how to hide/remove a upload/remove button from managed_file uplaod field. May be its seems a bit crazy, but my requirement was like that. Its sort of drupal theming and need to do in your drupal theme’s template.php.
Below snippet will do the magic