Removing outdated reference.

Author: andrewbrownmsft

articles/iot-hub-device-update/toc.yml

@@ -6,8 +6,6 @@ items:
   items:
   - name: What is Device Update for IoT Hub?
     href: understand-device-update.md
-  - name: Device Update for IoT Hub public preview documentation
-    href: /previous-versions/azure/iot-hub-device-update/understand-device-update
   - name: Release notes and version history
     href: device-update-changelog.md
 - name: Tutorials