Commit 22aadf3 ("sql: Implement mx_update_job2 trigger")
eliminated the `INQ`->`CANCELLED` housekeeping trigger, claiming that
the job state is no longer set to `CANCELLED` by code.

This is incorrect, as update_job_status_cancelled_by_group() still sets
the job status to `CANCELLED` when a group is cancelled.

Modify update_job_status_cancelled_by_group() to set the `job_cancelled`
flag instead of the `job_status`. Leave the job status transition to the
trigger.

Exclude `ASSIGNED` jobs, as the daemon handles them. Also, remove
unnecessary checks for unset `host_hostname`, `server_id`, and
`host_pid`.