[PATCH][_Hashtable] Fix hash code cache usage

[Florian Weimer]

* Jonathan Wakely:

>> I still think this is more confusing than necessary.  If there isn't
>> some general rule that empty objects can be considered stateless, that
>> should be added somewhere, too. 8-)
>
> Could you submit an issue (or two)?
> https://cplusplus.github.io/LWG/lwg-active.html#submit_issue

Do you still need this because there is already a PR?

  [hash.requirements] clarify that Cpp17Hash does not imply stateless
  <https://github.com/cplusplus/draft/pull/7733>

Thanks,
Florian