$batch includes full form state in unneeded cases

Escaped me during #629794: Scaling issues with batch API :

Comment says :

// We need the full $form_state when either:
// - Some submit handlers were saved to be called during batch
//   processing. See form_execute_handlers().
// - The form is multistep.

and the corresponding code is

if ($batch['has_form_submits'] || !empty($form_state['rebuild']) || !empty($form_state['storage'])) {
  $batch['form_state'] = $form_state;
}

After #634440: Remove auto-rebuilding magic for $form_state['storage'] and a couple fellow $form_state / rebuild issues, !empty($form_state['storage']) does not indicate a multistep form, only !empty($form_state['rebuild']) does, so the 3rd clause in the test can go away.