MINOR: applet: remove the thread mask from appctx_new()

appctx_new() is exclusively called with tid_bit and it only uses the
mask to pass it to the accompanying task. There is no point requiring
the caller to know about a mask there, nor is there any point in
creating an applet outside of the context of its own thread anyway.
Let's drop this and pass tid_bit to task_new() directly.
diff --git a/src/peers.c b/src/peers.c
index 6a9b0df..8af7475 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -3183,7 +3183,7 @@
 	peer->last_hdshk = now_ms;
 	s = NULL;
 
-	appctx = appctx_new(&peer_applet, tid_bit);
+	appctx = appctx_new(&peer_applet);
 	if (!appctx)
 		goto out_close;