Delete a user role from database.
$role: A string with the role name, or an integer with the role ID.
function user_role_delete($role) {
  if (is_int($role)) {
    $role = user_role_load($role);
  }
  else {
    $role = user_role_load_by_name($role);
  }
  // If this is the administrator role, delete the user_admin_role variable.
  if ($role->rid == variable_get('user_admin_role')) {
    variable_del('user_admin_role');
  }
  db_delete('role')
    ->condition('rid', $role->rid)
    ->execute();
  db_delete('role_permission')
    ->condition('rid', $role->rid)
    ->execute();
  // Update the users who have this role set:
  db_delete('users_roles')
    ->condition('rid', $role->rid)
    ->execute();
  module_invoke_all('user_role_delete', $role);
  // Clear the user access cache.
  drupal_static_reset('user_access');
  drupal_static_reset('user_role_permissions');
}