nix-cache: remove unused Gitea Actions runner

The actions runner on nix-cache01 was never actively used.
Removing it before migrating to nix-cache02.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hosts/nix-cache01/default.nix

@@ -3,6 +3,5 @@
   imports = [
     ./configuration.nix
     ../../services/nix-cache
-    ../../services/actions-runner
   ];
 }