Remove tests-nightly.yaml
Remove tests-nightly.yaml

This workflow has been disabled for ~6 months. Let's remove it for now,
we can add it back when/if we want to re-enable it.

Ref: https://github.com/cilium/cilium/actions/workflows/tests-nightly.yaml

Signed-off-by: Michi Mutsuzaki <a75494f12f73453b2bd75573f87b8f21fcc50565@isovalent.com>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium