remove all submenus from plugin

Not without touching the global $submenu directly (which I wouldn’t recommend – using only purposefully exposed APIs provides, in general, better stability).

However, if you wanted a function to remove all submenus:

/**
 * Removes all submenus of given menu item
 *
 * @param string $menu_slug The slug of the parent menu to remove submenus of
 */
function wpse137406_remove_submenus( $menu_slug ) {
    global $submenu;

    if ( isset( $submenu[$menu_slug] ) ){
         unset( $submenu[$menu_slug] );
    }
}