CLN: remove _igetitem_cache
Only used once (recent bugfixes got rid of the user uses IIRC) and its just an extra layer that isnt needed.
> Has this a performance implication?

The only user-facing paths affected are `iat.__setitem__` and `at.__setitem__`.  With unique columns this is 3x slower because we are no longer caching a Series object.  With non-unique columns this is ~4% faster because of a skipped check.  The benefit is in getting rid of the code path.