The manual sys.modules restoration code was redundant because
patch.dict.stop() automatically handles the cleanup. This simplifies
the tearDown method and removes the now-unused _original_langfuse_module
instance variable.
Addresses review comment: https://github.com/BerriAI/litellm/pull/21214#pullrequestreview-3802348462
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>