unset description in user_profile_form
It is better to increase weight of your custom module for removing description of timezone field. Your code should be like below:

if($form_id == 'user_profile_form'){
  unset($form['account']['mail']['#description']);
  unset($form['account']['pass']['#description']);
  unset($form['locale']['language']['#description']);
  unset($form['picture']['picture_delete']['#description']);
  unset($form['picture']['picture_upload']['#description']);
  unset($form['timezone']['timezone']['#description']);
}