samples: http_server: consolidate certificate options

Remove the CONFIG_NET_SAMPLE_CERTS_WITH_SC option and make the CA-signed
certificate the only option - there is no real downside to this over
using the unsigned certificate.

Remove adding of CA certificate as a TLS credential on the server, since
this credential is not used by the server. It may be useful to include
in any client code used to communicate with the server, so the
certificate itself is retained.

After this, some TLS tag enumerations are unused so have been removed.

Signed-off-by: Matt Rodgers <mrodgers@witekio.com>