MINOR: applet: Let the frontend appctx release the session

The session created for frontend applets is now totally owns by the
corresponding appctx. It means the appctx is now responsible to release
it. This removes the hack in stream_free() about frontend applets to be sure
to release the session.
diff --git a/src/dns.c b/src/dns.c
index bfd22c1..716514b 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -934,9 +934,10 @@
 		ha_alert("out of memory in dns_session_create().\n");
 		goto out_free_appctx;
 	}
+	appctx->sess = sess;
 
 	if (!sockaddr_alloc(&addr, &ds->dss->srv->addr, sizeof(ds->dss->srv->addr)))
-		goto out_free_sess;
+		goto out_free_appctx;
 
 	cs = cs_new_from_endp(appctx->endp, sess, &BUF_NULL);
 	if (!cs) {
@@ -965,8 +966,6 @@
 	/* Error unrolling */
  out_free_addr:
 	sockaddr_free(&addr);
- out_free_sess:
-	session_free(sess);
  out_free_appctx:
 	appctx_free(appctx);
  out_close: