destroy hetzner-hel1-01 (nagios) virtual machine at hetzner cloud

in #40695 (closed), we retired nagios, which involved cleaning up a lot of stuff and shutting down the VM, but we haven't destroyed it. normally, our retirement procedures schedule the VM's destruction in Ganeti, but this VM is in hetzner cloud so we can't automate that.

so, when this ticket comes to its due date (in one month), destroy the VM.