UserBackend: Drop abstract method hasUser

refs #8826

library/Icinga/Authentication/User/UserBackend.php

@@ -144,15 +144,6 @@ abstract class UserBackend implements Countable
         return $backend;
     }
 
-    /**
-     * Test whether the given user exists
-     *
-     * @param   User $user
-     *
-     * @return  bool
-     */
-    abstract public function hasUser(User $user);
-
     /**
      * Authenticate
      *