function deleteInactiveUser(inactiveUsers) {
  if (inactiveUsers.length > 0) {
    const userToDelete = inactiveUsers.pop();
    
    // Delete the inactive user.
    return admin.auth().deleteUser(userToDelete.uid).then(() => {
      return functions.logger.log(
        'Deleted user account',
        userToDelete.uid,
        'because of inactivity'
      );
    }).catch((error) => {
      return functions.logger.error(
        'Deletion of inactive user account',
        userToDelete.uid,
        'failed:',
        error
      );
    });
  } else {
    return null;
  }
}
Deletes one inactive user from the list.