[actions] remove magic-nix-cache

magic-nix-cache is EOL for non-enterprise Github users. I don't
intend to upgrade to Github enterprise or FlakeHub Cache, therefor
I remove it.

See DeterminateSystems/magic-nix-cache#119
for details.

.github/workflows/deploy-server.yml

@@ -56,9 +56,6 @@ jobs:
         with:
           nix_path: nixpkgs=channel:nixos-24.05
 
-      - name: Use magix nix cache
-        uses: DeterminateSystems/magic-nix-cache-action@87b14cf437d03d37989d87f0fa5ce4f5dc1a330b # v8
-
       - name: Check flake lock file
         uses: DeterminateSystems/flake-checker-action@078f5f7f47ee188aa6cb472527ca5984e195222d # v9
         with: