They are no longer used.
This removes a couple file_handle_cache:info/1 calls.
We are not removing them from the HTTP API to avoid
breaking things unintentionally.
Stats were not removed, including management UI stats
relating to FDs.
Web-MQTT and Web-STOMP configuration relating to FHC
were not removed.
The file_handle_cache itself must be kept until we
remove CQv1.
This is a follow-up commit to the parent commit. To quote part of the
parent commit's message:
> Historically, this was the Mnesia directory. But semantically, this
> should be the reverse: RabbitMQ owns the data directory and Mnesia is
> configured to put its files there too.
Now all subsystems call `rabbit:data_dir/0`. They are not tied to Mnesia
anymore.