feat(wpcarro/diogenes): Nixify diogenes's Terraform configuration

TL;DR:
- Define googleCloudVM function to provision NixOS VMs on Google Cloud.
- Consume googleCloudVM in diogenes/default.nix
- Define README.md for basic usage instructions (subject to change).
- Delete diogenes's HCL
- Remove `diogenesSystem` from meta.targets

I'm still having trouble with DNS:
- I need to transfer the Google Domains config to Cloud DNS
- `host billandhiscomputer.com` is NXDOMAIN, so I don't trust my tf DNS config
- This is preventing me from getting SSL certs, which blocks my website, quassel

Change-Id: If315876c96298e83a5953f13b62784d2f65a1024
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4747
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>

users/wpcarro/nixos/diogenes/README.md

@@ -0,0 +1,17 @@
+# diogenes
+
+diogenes is a NixOS machine deployed on a Google VM. It hosts
+https://wpcarro.dev.
+
+## Deployment
+
+I manage diogenes's deployment with Terraform. My current workflow looks like
+this (highly subject to change):
+
+```shell
+cd /tmp/terraform # or any directory that hosts terraform state
+outpath=$(nix-build /depot -A users.wpcarro.nixos.diogenes)
+cp <out-path> .
+nix-shell -p terraform google-cloud-sdk # gcloud to authenticate if necessary
+terraform init/apply
+```