s3:lib:tls: Use better priority lists for modern GnuTLS

s3:lib:tls: Use better priority lists for modern GnuTLS

We should use the default priority list. That is a good practice,
because TLS protocol hardening and phasing out of legacy algorithms,
is easier to co-ordinate when happens at a single place. See crypto
policies of Fedora.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14408

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Wed Jun 17 17:42:02 UTC 2020 on sn-devel-184