WebUI: prefer arrow functions whenever applicable

Compared to plain function, arrow function is simpler to understand (without bindings to `this`, `arguments`, `super`) and to read.
Now, plain function will only be used when this object is required.

PR #21691.

src/webui/www/private/confirmtrackerdeletion.html

@@ -30,7 +30,7 @@
                         hash: "*",
                         urls: urls,
                     },
-                    onComplete: function() {
+                    onComplete: () => {
                         window.parent.qBittorrent.Client.closeWindows();
                     }
                 }).send();