Remove GTlsConnection::need-certificate

Remove GTlsConnection::need-certificate

Trying to do this as a signal won't work well with either
GTlsCertificateDB (in which case looking up a certificate in the db is
a blocking/asynchronous act) or session resumption support (in which
case the certificate or lack thereof is part of the session definition
and so needs to be known immediately). Make the caller use
g_tls_connection_set_certificate() ahead of time (or when retrying)
instead.