fix: remove registry credentials from deploy step (public image)

Once maindoc-backend package is set to public on GHCR, Azure Container
Apps can pull without authentication — no GHCR_PAT needed in the deploy
step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: wisedev

.github/workflows/deploy-backend.yml

@@ -73,6 +73,3 @@ jobs:
           resourceGroup: MaIN
           containerAppName: maindoc-backend
           imageToDeploy: ${{ needs.build-push.outputs.image_tag }}
-          registryUrl: ghcr.io
-          registryUsername: ${{ github.actor }}
-          registryPassword: ${{ secrets.GHCR_PAT }}