test/k8sT: fix failing missed k8s tests
test/k8sT: fix failing missed k8s tests

* Remove checking the presence of manifest file using os.Stat this does
  not work for some reason when running missed k8s tests. Instead create
  a dummy file if it does not exist and run `kubectl delete` on that so
  if the file does not exist it won't throw the kubectl error.
* Remove dead code where we fallback to deleting `cilium.yaml` instead of
  timestamped file that we now use everywhere.

Signed-off-by: Deepesh Pathak <f919440c3f96a4d11125a0337b02a5a6cc587b4e@gmail.com>

michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,cilium-team/cilium,tgraf/cilium,tgraf/cilium,cilium-team/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,cilium/cilium