fix redundant exit message

[juliangruber]

Remove the exit event handler which would create duplicate activity events on a crash.

[bajtos]

IIUC, your change is removing redundant activity log entries printed when the Zinnia process exits either with a non-zero exit code or via a signal. After your change, only one activity should be logged. That's a great improvement! 👏🏻

[juliangruber]

https://github.com/filecoin-station/core/releases/tag/v21.1.3