Clean up VirtualBox demo appliance residue (#354282)

Author: emilazy

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -218,6 +218,9 @@
 
 - `androidenv.androidPkgs_9_0` has been removed. It is replaced with `androidenv.androidPkgs` for a more complete Android SDK, including support for Android 9 and later.
 
+- The VirtualBox demo installer appliance has been removed.
+  Please use the standard installer ISOs instead.
+
 - `grafana` has been updated to version 11.1. This version doesn't support setting `http_addr` to a hostname anymore, an IP address is expected.
 
 - `deno` has been updated to Deno 2, which has breaking changes.