Drupal 8 : How to remove class of form field?

Drupal form is generating below HTML

 <input class="btn--primary  button js-form-submit form-submit btn" data-drupal-selector="edit-next" id="edit-next" name="firstName" value="Submit" type="submit">

I would like to remove button class from that.

How can I achieve that in hook_form_FORM_ID_alter()?

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/4/feed