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

CLAUDE.md

@@ -130,10 +130,10 @@ removing a host.
   host be built as a standalone, `qm importdisk`-ready `.raw` image with no
   install step — see `docs/proxmox-images.md`.
 - `modules/boot/efi.nix` — systemd-boot + EFI vars, paired with the disko module.
-- `modules/installer/` — the auto-installer environment (ISO/netboot/Proxmox
-  LXC): `common.nix` (shared config + the generated `auto-install.sh`),
-  `iso.nix`/`proxmox-lxc.nix` (per-variant), `host-keys.nix` (optionally
-  bakes `host-keys/` into the image under `--impure`). See
+- `modules/installer/` — the auto-installer environment (ISO, also served as
+  PXE netboot): `common.nix` (shared config + the generated
+  `auto-install.sh`), `iso.nix`, `host-keys.nix` (optionally bakes
+  `host-keys/` into the image under `--impure`). See
   `docs/auto-installer.md`.
 - `modules/pxe-boot/stage-installer-artifacts.nix` — builds the installer's
   netboot image and stages it on the `pxe-boot` host so its iPXE menu can