Remove some remnants from http/2 refactoring

HttpTransact::handle_response_keep_alive_headers(State *s, HTTPVersion ver, HTTPHdr *heads)
{
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.
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    // check that the client is HTTP 1.1 and the conf allows chunking
    if (s->client_info.http_version == HTTPVersion(1, 1) &&
        (s->txn_conf->chunking_enabled == 1 ||
         (s->state_machine->plugin_tag &&
          (!strcmp(s->state_machine->plugin_tag, "http/2") || !strncmp(s->state_machine->plugin_tag, "spdy", 4)))) &&
        // if we're not sending a body, don't set a chunked header regardless of server response
        !is_response_body_precluded(s->hdr_info.client_response.status_get(), s->method) &&
        // we do not need chunked encoding for internal error messages