Remove `-Zkeep-hygiene-data`.
It was added way back in #28585 under the name `-Zkeep-mtwt-tables`. The justification was:
> This is so that the resolution results can be used after analysis,
> potentially for tool support.
There are no uses of significance in the code base, and various Google searches for both option names (and variants) found nothing of interest. I think this can safely be removed.
r? `@davidtwco`