Remove thumbnail generation task

Now that the thumbnail generation task has been ported to a worker and that we dropped the code that calls the task, we can finish the job.

changed milestone to %Arkindex 1.6.0

requested review from @babadie

assigned to @erouchet

added 2 commits

• 51d01801 - 1 commit from branch master
• ae518f61 - Remove thumbnail generation task

Compare with previous version

Congrats ! Always happy to remove some old code

enabled an automatic merge when the pipeline for ae518f61 succeeds

merged