Add the snippet below to your themes functions.php file to remove ALL of the widgets from the WordPress admin dashboard.

remove_action('welcome_panel', 'wp_welcome_panel');
function remove_dashboard_widgets() {
    global $wp_meta_boxes;
    unset($wp_meta_boxes['dashboard']['normal']['core']['dashboard_activity']);
    unset($wp_meta_boxes['dashboard']['normal']['core']['dashboard_incoming_links']);
    unset($wp_meta_boxes['dashboard']['normal']['core']['dashboard_recent_comments']);
    unset($wp_meta_boxes['dashboard']['normal']['core']['dashboard_plugins']);
    unset($wp_meta_boxes['dashboard']['normal']['core']['dashboard_right_now']);
    unset($wp_meta_boxes['dashboard']['side']['core']['dashboard_recent_drafts']);
    unset($wp_meta_boxes['dashboard']['side']['core']['dashboard_quick_press']);
    unset($wp_meta_boxes['dashboard']['side']['core']['dashboard_primary']);
    unset($wp_meta_boxes['dashboard']['side']['core']['dashboard_secondary']);
    update_user_meta(get_current_user_id(), 'show_welcome_panel', false);
}
add_action('wp_dashboard_setup', 'remove_dashboard_widgets' );