Don't forcibly downgrade context in WP_REST_Users_Controller::get_items() #1869

In WP_REST_Users_Controller::get_items(), we forcibly downgrade context if the user doesn't have list_users capability:

if ( ! current_user_can( 'list_users' ) ) {
    $prepared_args['has_published_posts'] = true;

    // Only display a public subset of information
    $request['context'] = 'embed';
}

This is inconsistent with the rest of our controllers. If the current user can't access the supplied context, we should return an error.

Related #1845