incus: remove subuid / subgid creation from sysext - #3028

incus: remove subuid / subgid creation from sysext

This create more issues than it solves:

• override existing subuid / subgid
• not flexible for the end user
• it has to be created only once (while tmpfiles always try to create
  those files)

I think Flatcar should not be responsible to create this and it should
be documented (flatcar/flatcar-website#441) on how to do it through Ignition:

version: 1.1.0
variant: flatcar
storage:
  files:
    - path: /etc/subuid
      append:
        - inline: |
            root:1065536:65536
    - path: /etc/subgid
      append:
        - inline: |
            root:1065536:65536

Tested with the produced image:

core@localhost ~ $ cat /etc/subuid
foo:100000:65536
root:1065536:65536

with:

# config.yaml
# butane < config.yaml > config.json
variant: flatcar
version: 1.1.0
passwd:
  users:
    - name: foo
storage:
  files:
    - path: /etc/subuid
      append:
        - inline: |
            root:1065536:65536
    - path: /etc/subgid
      append:
        - inline: |
            root:1065536:65536
    - path: /etc/flatcar/enabled-sysext.conf
      contents:
        inline: |
          incus

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.
• CI: http://localhost:8080/job/container/job/packages_all_arches/6163/cldsv/

I think we should do the same for Podman, but we need to think about a migration path as it will break existing workload.