Remove packages.all bundle and the installer's own proxmox-lxc target

Both existed only so the installer could boot as an LXC container and
nixos-install some other host from within it, but lxc-* targets are
already excluded from the install menu (nixos-install can't touch its
own running root filesystem), and now have their own direct tarball
path anyway. That left the installer's own LXC form with no real use
case, and packages.all with only two members worth bundling.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

scripts/prepare-host-key.sh

@@ -62,8 +62,8 @@ Generated: ${keyfile}(.pub)
   a) Rebuild the installer image with all host-keys/ baked in (see
      docs/auto-installer.md):
        NIXOS_HOST_KEYS_DIR="${keydir}" nix build .#iso --impure
-     (or .#lxc / .#pxe / .#all — --impure is required since host-keys/
-     is gitignored and flakes can't see it otherwise)
+     (or .#pxe — --impure is required since host-keys/ is gitignored and
+     flakes can't see it otherwise)
 
   b) Or, for an image already built without keys, scp it in after boot:
        scp ${keyfile}{,.pub} root@<target-ip>:/root/host-keys/