chore(setup_helpers): remove dead cpp_flag_cache and fix parameter typo (#6085)

Remove the module-level `cpp_flag_cache = None` variable and its stale
comment "Every call will cache the result". The actual caching is handled
by the `@lru_cache` decorator on `auto_cpp_level` directly below.

Also rename the typo'd parameter `obg` to `obj` in `no_recompile`.
Nothing passes this argument by keyword, so there is no API break.

Part of #6084

Assisted-by: ClaudeCode:claude-sonnet-4-6