s4:rpc_server/netlogon: if we require AES there's no need to remove the ARCFOUR flag

s4:rpc_server/netlogon: if we require AES there's no need to remove the ARCFOUR flag

With SAMBA_WEAK_CRYPTO_DISALLOWED dcesrv_netr_ServerAuthenticate3_check_downgrade()
will return DOWNGRADE_DETECTED with negotiate_flags = 0, if AES was not
negotiated...

And if AES was negotiated there's no harm in returning the ARCFOUR
flag...

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15425

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>