rephist: Remove unused counters in or_history_t

The rephist subsystem seems to track EXTEND cells attempt (on the client side) but the overall results of this is not used except in rep_hist_dump_stats(). This is the only use of link_history_t also afaict. See:

void rep_hist_note_extend_succeeded(const char *from_name,
                                    const char *to_name);
void rep_hist_note_extend_failed(const char *from_name, const char *to_name);

Furthermore, tor tracks downtime and uptime of relays but actually never use that information anywhere. See:

void rep_hist_note_connect_failed(const char* nickname, time_t when);
void rep_hist_note_connect_succeeded(const char* nickname, time_t when);
void rep_hist_note_disconnect(const char* nickname, time_t when);
void rep_hist_note_connection_died(const char* nickname, time_t when);

The side effect of this is that we keep adding objects to the history_map that are wasting memory and never used in the end except when we dump statistics.

By removing this, we would cleanup quite a bit of code and effectively make the or_history_t object ONLY useful to directory authorities for relay reachability tracking.