Drop example user backend

We already provide an interface for application developers, this file is outdated and thus should get removed.

Addresses No. 3 from https://github.com/owncloud/core/issues/14847

lib/private/user/example.php

-<?php
-
-/**
- * ownCloud
- *
- * @author Frank Karlitschek
- * @copyright 2012 Frank Karlitschek frank@owncloud.org
- *
- * This library is free software; you can redistribute it and/or
- * modify it under the terms of the GNU AFFERO GENERAL PUBLIC LICENSE
- * License as published by the Free Software Foundation; either
- * version 3 of the License, or any later version.
- *
- * This library is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU AFFERO GENERAL PUBLIC LICENSE for more details.
- *
- * You should have received a copy of the GNU Affero General Public
- * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
- *
- */
-
-/**
- * abstract reference class for user management
- * this class should only be used as a reference for method signatures and their descriptions
- */
-abstract class OC_User_Example extends OC_User_Backend {
-	/**
-		* Create a new user
-		* @param string $uid The username of the user to create
-		* @param string $password The password of the new user
-		* @return bool
-		*
-		* Creates a new user. Basic checking of username is done in OC_User
-		* itself, not in its subclasses.
-		*/
-	abstract public function createUser($uid, $password);
-
-	/**
-		* Set password
-		* @param string $uid The username
-		* @param string $password The new password
-		* @return bool
-		*
-		* Change the password of a user
-		*/
-	abstract public function setPassword($uid, $password);
-
-	/**
-		* Check if the password is correct
-		* @param string $uid The username
-		* @param string $password The password
-		* @return string
-		*
-		* Check if the password is correct without logging in the user
-		* returns the user id or false
-		*/
-	abstract public function checkPassword($uid, $password);
-
-	/**
-		* get the user's home directory
-		* @param string $uid The username
-		* @return string
-		*
-		* get the user's home directory
-		* returns the path or false
-		*/
-	abstract public function getHome($uid);
-}