virnettlscontext: Don't set DH parameters ourselves

virnettlscontext: Don't set DH parameters ourselves

According to [1]:

  Prior to GnuTLS 3.6.0 for the ephemeral or anonymous
  Diffie-Hellman (DH) TLS ciphersuites the application was
  required to generate or provide DH parameters. That is no
  longer necessary as GnuTLS utilizes DH parameters and
  negotiation from [RFC7919].

This allows us to:

  a) drop the code that's setting DH params,
  b) drop @dhParams member from _virNetTLSContext struct. and
  c) drop gnutls_dh_params_generate2() mock.

1: https://www.gnutls.org/manual/html_node/Parameter-generation.html

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>