MINOR: http_fetch/htx: Use stream flags instead of px mode in smp_prefetch_htx

In the function smp_prefetch_htx(), we must know if data in the channel's buffer
are structured or not. Before the proxy mode was tested. Now we test if the
stream is an HTX stream or not. If yes, we know the HTX is used to structure
data in the channel's buffer.
diff --git a/src/http_fetch.c b/src/http_fetch.c
index 51f2ef1..d720b37 100644
--- a/src/http_fetch.c
+++ b/src/http_fetch.c
@@ -169,7 +169,6 @@
  */
 struct htx *smp_prefetch_htx(struct sample *smp, const struct arg *args)
 {
-	struct proxy *px = smp->px;
 	struct stream *s = smp->strm;
 	unsigned int opt = smp->opt;
 	struct http_txn *txn = NULL;
@@ -190,7 +189,7 @@
 		txn = s->txn;
 	}
 
-	if (px->mode == PR_MODE_HTTP) {
+	if (IS_HTX_STRM(s)) {
 		if ((opt & SMP_OPT_DIR) == SMP_OPT_DIR_REQ) {
 			htx = htxbuf(&s->req.buf);
 			if (htx_is_empty(htx) || htx_get_tail_type(htx) < HTX_BLK_EOH) {
@@ -221,7 +220,7 @@
 			}
 		}
 	}
-	else { /* PR_MODE_TCP */
+	else { /* RAW mode */
 		if ((opt & SMP_OPT_DIR) == SMP_OPT_DIR_REQ) {
 			struct buffer *buf;
 			struct h1m h1m;