Task, dynamic group, repeat message at each cron run

[ddurieux]

When a computer is no longer in a dynamic group, the message Device no longer defined in definition of job as added at each execution of the cron (taskscheduler).

We need not add the message if added previously.

[GuillaumeV-cemea]

On 9.5+3.0, the problem is still present (the code changed quite a bit, I haven't dissected to find the root cause of the issue), however applying the proposed fix in #3031 works - no more spamming of the message "Device no longer defined in definition of job"