segment_profiling: Remove SegmentProfiler and load_sample event

While it seems interesting functionality, this hasn't been documented,
maintained or knowingly leveraged for many years.

There are various other approaches today, too:

* We track the number of event handler invocations regardless of
  profiling. It's possible to approximate a load_sample event by
  comparing the result of two get_event_stats() calls. Or, visualize
  the corresponding counters in a Prometheus setup to get an idea of
  event/s broken down by event names.

* HookCallFunction() allows to intercept script execution, including
  measuring the time execution takes.

* The global call_stack and g_frame_stack can be used from plugins
  (and even external processes) to walk the Zeek script stack at certain
  points to implement a sampling profiler.

* USDT probes or more plugin hooks will likely be preferred over Zeek
  builtin functionality in the future.

Relates to #3458

src/Event.cc

@@ -120,8 +120,6 @@ void EventMgr::Drain() {
     if ( event_queue_flush_point )
         Enqueue(event_queue_flush_point, Args{});
 
-    detail::SegmentProfiler prof(detail::segment_logger, "draining-events");
-
     PLUGIN_HOOK_VOID(HOOK_DRAIN_EVENTS, HookDrainEvents());
 
     draining = true;