MINOR: htx: Remove the macro IS_HTX_SMP() and always use IS_HTX_STRM() instead

The macro IS_HTX_SMP() is only used at a place, in a context where the stream
always exists. So, we can remove it to use IS_HTX_STRM() instead.
diff --git a/src/payload.c b/src/payload.c
index a16f8c6..f9b5fe0 100644
--- a/src/payload.c
+++ b/src/payload.c
@@ -55,7 +55,7 @@
 
 	chn = ((smp->opt & SMP_OPT_DIR) == SMP_OPT_DIR_RES) ? &smp->strm->res : &smp->strm->req;
 	smp->data.type = SMP_T_SINT;
-	if (IS_HTX_SMP(smp)) {
+	if (IS_HTX_STRM(smp->strm)) {
 		struct htx *htx = htxbuf(&chn->buf);
 		smp->data.u.sint = htx->data - co_data(chn);
 	}