MEDIUM: backend: don't rely on mux_pt_ops in connect_server()

The comment above the change remains true. We assume there is always 1
conn_stream per outgoing connectionq. Today, it is always true because H2 is not
supported yet for server connections.
diff --git a/src/backend.c b/src/backend.c
index a52eacc..50ef3b7 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1123,7 +1123,7 @@
 			 */
 			srv_cs = srv_conn->mux_ctx;
 
-			if (srv_conn->mux == &mux_pt_ops && srv_cs->data) {
+			if (srv_cs->data) {
 				si_detach_endpoint(srv_cs->data);
 				if (old_cs && !(old_cs->conn->flags & CO_FL_PRIVATE)) {
 					si_attach_cs(srv_cs->data, old_cs);