Implement hook_uninstall to delete the variables

add the following code in file_upload_security.install

/**
 * @file
 * Install, update and uninstall functions for the file_upload_security module.
 */

/**
 * Implements hook_uninstall().
 */
function file_upload_security_uninstall() {
  variable_del('file_upload_security_fix');
  variable_del('file_upload_security_level')
}