Remove special case for TLS 1.3.

We now set the server certificate in tls_choose_sigalg() so there is
no need for a special case for TLS 1.3 any more.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2339)