cleanup: remove stale local/cache pkgar and rbos-cache artifacts

These are leftover cache artifacts from earlier build experiments.
The new content-hash-based cache system stores dep_hashes.toml in
per-recipe target/ dirs and repo/ binary store, not in local/cache/.

local/cache/pkgar/base/auto_deps.toml

@@ -1 +0,0 @@
-packages = []

local/cache/rbos-cache-20260428-080713/base-initfs.deps

@@ -1 +0,0 @@
-packages = []

local/cache/rbos-cache-20260428-080713/base.deps

@@ -1 +0,0 @@
-packages = []