actors: Remove <hive-proxy>; no longer export actor-hive.

The goal of hive-proxy (in XUDD at least) was to prevent actors from
accessing their hive.  Since we're no longer exporting actor-hive
we've more or less achieved that though.

* 8sync/systems/actors.scm: No longer export actor-hive.
  (<message>): Adjust comment about deferred-reply.
  (<hive-proxy>, <debug-hive-proxy>): Removed.