Bug in civicrm_group_roles.module - deleting Contact from Group doesn't remove corresponding permission role

We use the civicrm_group_roles.module included with CiviCRM to map permissions from CiviCRM Groups to Drupal roles. We discovered that deleting a Contact from a Group fails to remove the corresponding Drupal permission role.

This patch fixes the issue (for the Drupal 6 version of the module, not sure if the same problem applies to Drupal 7).