ldap: add users_get_handle_no_user()
    
    If a user is not found on the server anymore but still has a cache entry
    this entry should be removed. The code to remove a group is already
    moved into a separate function groups_get_handle_no_group(). With this
    patch the code to remove a user is moved into a similar function so that
    it can be reused at other places.
    
    Related to https://pagure.io/SSSD/sssd/issue/3984
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>