introducer: stop tracking hints for subscribed clients

introducer: stop tracking hints for subscribed clients

A long time ago, the introducer's status web page would show the
advertised IP addresses for all subscribers, by parsing their
RemoteReference's FURL's connection hints. This hasn't worked since
about 12-Aug-2014 when foolscap-0.6.5 changed the internal format of
these hints.

This removes the feature: we no longer attempt to show advertised IP
addresses of subscribed clients. It also removes the code that looked
inside foolscap internals for this information.