Don't unset the job submit ID when a task is done.

This caused occasional failures in the events/task test suite: if a task
reported in succeeded while a poll command was in progress (due to 
an execution timeout), the poll result would set the task back to 'running',
but the 'task succeeded' message just prior to that had unset the submit 
ID thereby preventing the next poll from correcting the problem.

Also removed a little redundant code in there.