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 on how to do it through Ignition:
```yaml
version: 1.1.0
variant: flatcar
storage:
  files:
    - path: /etc/subuid
      append:
        - inline: |
            root:1065536:65536
    - path: /etc/subgid
      append:
        - inline: |
            root:1065536:65536
```

Signed-off-by: Mathieu Tortuyaux <[email protected]>

Author: tormath1

build_library/sysext_mangle_flatcar-incus

@@ -14,8 +14,6 @@ mkdir -p ./usr/lib/tmpfiles.d
 pushd ./usr/lib/tmpfiles.d
 cat <<EOF >./10-incus.conf
 d  /var/lib/lxc/rootfs	0755  root  root  -  -
-w+ /etc/subuid - - - - root:1065536:65536
-w+ /etc/subgid - - - - root:1065536:65536
 EOF
 popd