Don't create/update Worker row when starting scheduler/provisioner

I noticed that the row in the Worker table for the Celery worker that runs server tasks had its concurrency incorrectly set to 1, which is because we were accidentally setting it when starting either the scheduler or the provisioner service. Don't do that.

Fixes the remaining part of #751 (closed).

As a practical matter, this shouldn't be merged until !1793 (merged) has been merged, because the incorrect concurrency happened to accidentally work around that bug much of the time.