Remove cache_peer_domain directive

Remove cache_peer_domain directive

Identical functionality is provided through cache_peer_access.

While this check appears at face value to be simpler than ACLs, the
reality is that:
* the difference is simply the time it takes to initialize and destruct
  an on-stack Checklist,
* processing the checks may take longer than ACLs (linked-list of string
  comparisons vs single tree lookup),
* ACLs are the common case due to their extra flexibility, and
* extra work is being done per-transaction just to check which of the
  two features is in use.

By removing we gain less code and configuration directives to work
around in the long term.