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, 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.
Merge pull request #492 from hjoliver/no-submit-id-unset

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