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

pybind11/setup_helpers.py

diff --git a/pybind11/setup_helpers.py b/pybind11/setup_helpers.py
index f242918..300688b 100644
--- a/pybind11/setup_helpers.py
+++ b/pybind11/setup_helpers.py
@@ -245,10 +245,6 @@
         return True
 
 
-# Every call will cache the result
-cpp_flag_cache = None
-
-
 @lru_cache
 def auto_cpp_level(compiler: Any) -> str | int:
     """
@@ -338,7 +334,7 @@
     return os.stat(obj).st_mtime < os.stat(src).st_mtime
 
 
-def no_recompile(obg: str, src: str) -> bool:  # noqa: ARG001
+def no_recompile(obj: str, src: str) -> bool:  # noqa: ARG001
     """
     This is the safest but slowest choice (and is the default) - will always
     recompile sources.