s3:winbind: Remove no longer used domain's private_data pointer

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15046

Signed-off-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>