[AzureAD-SSPR] Remove retired article from TOC

Author: iainfoulds

articles/active-directory/authentication/TOC.yml

@@ -68,8 +68,6 @@
       href: howto-sspr-deployment.md
     - name: Pre-register authentication data
       href: howto-sspr-authenticationdata.md
-    - name: Enable password writeback
-      href: howto-sspr-writeback.md
     - name: SSPR for Windows clients
       href: howto-sspr-windows.md
   - name: Cloud-based MFA