selinux: Delete existing user mapping on empty default
    
    https://fedorahosted.org/sssd/ticket/2587
    
    The case of SELinux default user mapping being an empty string is valid,
    it should translate into "pick the default context on the target
    machine".
    
    In case the context is empty, we need to delete the per-user mapping from
    the SELinux database to make sure the default is used.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Pavel Reichl <preichl@redhat.com>