CLEANUP: conn_stream: merge cs_new_from_{mux,applet} into cs_new_from_endp()

The two functions became exact copies since there's no more special case
for the appctx owner. Let's merge them into a single one, that simplifies
the code.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 6cb2334..9ed2727 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -1629,7 +1629,7 @@
 	 */
 	sess->t_idle = tv_ms_elapsed(&sess->tv_accept, &now) - sess->t_handshake;
 
-	if (!cs_new_from_mux(h2s->endp, sess, input))
+	if (!cs_new_from_endp(h2s->endp, sess, input))
 		goto out_close;
 
 	h2c->nb_cs++;