kubectl-example: Remove empty vendor dependency

pkgs/applications/networking/cluster/kubectl-example/default.nix

···
     sha256 = "18vp53cda93qjssxygwqp55yc80a93781839gf3138awngf731yq";
   };
 
-  vendorSha256 = "0sjjj9z1dhilhpc8pq4154czrb79z9cm044jvn75kxcjv6v5l2m5";
+  vendorSha256 = null;
 
   meta = with lib; {
     description = "kubectl plugin for retrieving resource example YAMLs";