base-initfs: drop ion (initfs recovery shell) to move forward with nix

ion pulls nix 0.23.1 (git), which has the Redox SaFlags_t=c_ulong bug
(relibc/libc use c_int) and fails to compile on a clean rebuild with the
`kill`/signal feature. ion was only a bundled initfs recovery shell that
nothing execs (init runs daemons then switchroots to /usr, where the
interactive login shell is brush). Per operator directive, drop ion rather
than hold nix back on an ancient pinned version for its sake.

recipes/core/base-initfs/recipe.toml

@@ -8,7 +8,6 @@ version = "0.3.1"
 template = "custom"
 dependencies = [
   "redoxfs",
-  "ion",
   "driver-manager",
   # userutils provides `getty`, copied into the bare initfs from the
   # sysroot below. Without this dependency getty is absent from the
@@ -133,7 +132,9 @@ do
 done
 
 cp "${COOKBOOK_SYSROOT}/usr/bin/redoxfs" "${COOKBOOK_BUILD}/initfs/bin"
-cp "${COOKBOOK_SYSROOT}/usr/bin/ion" "${COOKBOOK_BUILD}/initfs/bin"
+# ion removed: it was only a bundled initfs recovery shell (nothing execs it);
+# not worth holding nix back on old 0.23.1. The interactive login runs from
+# /usr (brush) after switchroot.
 cp "${COOKBOOK_SYSROOT}/usr/bin/driver-manager" "${COOKBOOK_BUILD}/initfs/bin"
 # NB: getty/login are intentionally NOT copied into the initfs. The
 # interactive login runs from /usr after switchroot (config/minimal.toml's