CLEANUP: applet: rename struct si_applet to applet

Since this one does not depend on stream_interface anymore, remove the
"si_" prefix.
diff --git a/include/types/obj_type.h b/include/types/obj_type.h
index aefbcd3..83a3e78 100644
--- a/include/types/obj_type.h
+++ b/include/types/obj_type.h
@@ -36,7 +36,7 @@
 	OBJ_TYPE_LISTENER,     /* object is a struct listener */
 	OBJ_TYPE_PROXY,        /* object is a struct proxy */
 	OBJ_TYPE_SERVER,       /* object is a struct server */
-	OBJ_TYPE_APPLET,       /* object is a struct si_applet */
+	OBJ_TYPE_APPLET,       /* object is a struct applet */
 	OBJ_TYPE_APPCTX,       /* object is a struct appctx */
 	OBJ_TYPE_CONN,         /* object is a struct connection */
 	OBJ_TYPE_ENTRIES       /* last one : number of entries */