Restrict category for custom post type in wordpress

I have created a custom post type for adding employees. I have added the category taxonomy for it. Then I have added a category named “Employees” (slug: employees). But when I add a new employee, it shows me all the categories to choose. How can I restrict it to show only “Employees” category here?

Here is my php code in functions.php:

function employees_register() {
    $labels = array(
        'name' => _x('Employees', 'post type general name'),
        'singular_name' => _x('Employee', 'post type singular name'),
        'add_new' => _x('Add New', 'employees'),
        'add_new_item' => __('Add New Employee'),
        'edit_item' => __('Edit Employee'),
        'new_item' => __('New Employee'),
        'view_item' => __('View Employee'),
        'search_items' => __('Search Employee'),
        'not_found' =>  __('Nothing found'),
        'not_found_in_trash' => __('Nothing found in Trash'),
        'parent_item_colon' => ''
    );

    $args = array(
        'labels' => $labels,
        'public' => true,
        'publicly_queryable' => true,
        'show_ui' => true,
        'query_var' => true,
        'taxonomies' => array( 'category' ),
        'rewrite' => true,
        'capability_type' => 'post',
        'hierarchical' => false,
        'menu_position' => null,
        'supports' => array('title','editor','thumbnail')
      ); 

    register_post_type( 'employees' , $args );
}
add_action('init', 'employees_register');