CLEANUP: muxes: Consider stream's sd as defined in .show_fd callback functions

In muxes, the stream-endoint descriptor of a stream is always defined. Thus,
in .show_fd callback functions, there is no reason to test it.

This patch should address the issue #1727.
.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 20c646f..bdd47f5 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -6767,9 +6767,7 @@
 				      h2s_sc(h2s)->flags, h2s_sc(h2s)->app);
 
 		chunk_appendf(msg, "sd=%p", h2s->sd);
-		if (h2s->sd)
-			chunk_appendf(msg, "(.flg=0x%08x)",
-				      se_fl_get(h2s->sd));
+		chunk_appendf(msg, "(.flg=0x%08x)", se_fl_get(h2s->sd));
 
 		chunk_appendf(&trash, " .subs=%p", h2s->subs);
 		if (h2s->subs) {