Remove support for max_threads event

After testing and reading the code it seems that max_threads trigger cannot be
possibly activated with the current design (i.e. spawn_thread is never called
to exceed max_threads).

It seems it cannot be triggered with any kind of load, and if it were
possible to trigger by invalid configuration (which was the case before the
check for start_threads <= max_threads was added), it wouldn't warrant a
trigger.

Still, I might be wrong, or there might be a plan to make it useful. In that
case, please feel free to reject the change. Thank you.