WebUI: simplify close window implementation

The caller site now take the responsibility to ensure the element is valid.

PR #21892.

src/webui/www/private/views/searchplugins.html

@@ -114,7 +114,7 @@
         };
 
         const closeSearchWindow = (id) => {
-            window.parent.MochaUI.closeWindow(window.parent.$(id));
+            window.parent.qBittorrent.Client.closeWindow(window.parent.$(id));
         };
 
         const installPlugin = (path) => {