code: @Override
      public void deleteUser(String username) throws AuthenticationException {
          String user = file_store.getProperty(username);
          if (user == null) {
              throw new AuthenticationException("User '" + username
                      + "' not found.");
          }
          file_store.remove(username);
          try {
              saveUsers();
          } catch (IOException e) {
              throw new AuthenticationException("Error deleting user: ", e);
          }
      }
docs: Delete a user.
  
  @param username The username of the user to delete.
  @throws AuthenticationException if there was an error during deletion.