desktop_notification: Fix bad rendering of math formulas.

Earlier, for the desktop notifications having latex math
like "$$1 \oplus 0 = 1$$, the notification had the math
included multiple times.

This commit fixes the incorrect behavior by replacing
the KaTeX with the raw LaTeX source.

Fixes #25289.

web/tests/notifications.test.js

@@ -340,6 +340,7 @@ test("message_is_notifiable", () => {
 
 test("basic_notifications", () => {
     $("<div>").set_find_results(".emoji", {replaceWith() {}});
+    $("<div>").set_find_results("span.katex", {each() {}});
 
     let n; // Object for storing all notification data for assertions.
     let last_closed_message_id = null;