function deleteInactiveUser(inactiveUsers) {
  if (inactiveUsers.length > 0) {
    const userToDelete = inactiveUsers.pop();

    // Delete the inactive user.
    return admin.auth().deleteUser(userToDelete.uid).then(() => {
      return logger.log(
          "Deleted user account",
          userToDelete.uid,
          "because of inactivity",
      );
    }).catch((error) => {
      return logger.error(
          "Deletion of inactive user account",
          userToDelete.uid,
          "failed:",
          error,
      );
    });
  } else {
    return null;
  }
}
Deletes one inactive user from the list.
@param {admin.auth.UserRecord[]} inactiveUsers
@return {null | Promise<void>}