[V5] Drop IJobNotificationHandler

[nulltoken]

I'm not really seeing the value of this anymore.

Given that

• IExceptionHandler already handles exceptions
• Some minor improvements to IJobExecutionProgressReporter may allow a client to register to filtered events. This would require at least the following:
  • Make ExecutionProgress expose types and names of jobs
  • Enhance IJobExecutionProgressReporter an additional method which could look like `IDisposable Register(FuncExecutionProgress, bool> filterby, <Action<ExecutionProgress> callback);

Those two things altogether could replace the current value that IJobNotificationHandler serves today.

[linkdotnet]

Just to give some rational: IJobNotificationHandler was my first attempt for "outside observability" and to model job dependencies.

Over time, it might be totally redundant. So yeah, let's remove that bid.