function plt_hide_custom_sidebars_metaboxes() {
$screen = get_current_screen();
if ( !$screen ) {
return;
}
//Hide the "Sidebars" meta box.
remove_meta_box('customsidebars-mb', $screen->id, 'side');
}
add_action('add_meta_boxes', 'plt_hide_custom_sidebars_metaboxes', 20);