Remove the drupal_flush_all_caches() from hook_update_N

Remove the drupal_flush_all_caches() from hook_update_N

Instead of flushing the caches on the hook_update_N, it better to Specify in Release notes saying "In this Release one need to Flush caches"

/**
 * Clear cache to notice the CTools load callback.
 */
function field_group_update_7008() {
  drupal_flush_all_caches();
}