Add hook_uninstall to delete RPX variables

/**
 * @file
 *
 * Deletes system variables set by the RPX module.
 */

/**
 * Implementation of hook_uninstall().
 */
function rpx_uninstall() {
  variable_del('rpx_realm');
  variable_del('rpx_apikey');
  variable_del('rpx_admin_url');
}

I had some problems with the rpx_realm being set to an invalid value. I thought I'd be able to uninstall the module to remove the bad values, but there was no option. This patch makes it possible to remove such bad values.