Remove preview branch workflow, replaced by existing nightly release

The preview branch accumulated dated firmware binaries in Git, causing
the repo to grow large and slow to clone. The nightly GitHub Release
already serves the same purpose and is what grid-editor uses to download
firmware. The preview branch and its workflow are no longer needed.

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

Author: SukuWc

.github/workflows/firmware_deploy_preview.yml

@@ -34,9 +34,3 @@ jobs:
       product_name: "knot"
     secrets: inherit
 
-  deploy-preview:
-    name: Deploy to Preview Branch
-    # Run on main branch OR when a tag exists to ensure preview is never behind production releases
-    if: github.ref == 'refs/heads/main' || needs.build.outputs.release_version != ''
-    needs: build
-    uses: ./.github/workflows/firmware_deploy_preview.yml