How to remove webform error message?
drupal_get_messages() returns all the messages set using drupal_set_message, which also includes error messages set during form validation. This also clears the messages.

You can then add a custom validate handler to the webform by altering it by implementing hook_form_alter or hook_form_FORM_ID_alter.

In your custom validate handler, call drupal_get_messages() to clear any error messages.