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

README.md

@@ -53,7 +53,7 @@ nix eval --json .#nixosConfigurations --apply builtins.attrNames | jq -r '.[]'
 | `modules/build-types/` | Build-type-specific config: what makes a system minimal/server/docker/gui/pxe-boot/nix-cache |
 | `modules/common/` | Shared NixOS config, Home Manager, aliases imported by every host |
 | `modules/nix-cache/` | Binary cache and remote builder client/server modules |
-| `modules/installer/` | Auto-installer environment (ISO/netboot/Proxmox LXC) — see `docs/auto-installer.md` |
+| `modules/installer/` | Auto-installer environment (ISO, also served as PXE netboot) — see `docs/auto-installer.md` |
 | `host-keys/` | Gitignored, locally-generated SSH host keys for the auto-installer — see `docs/auto-installer.md` |
 | `docs/` | Operational notes for cache, builders, lock updates, boot services, the auto-installer, and Proxmox image builds |
 | `scripts/` | Codex setup, validation, and host-key-prep helpers |