Drop WorkerRun TTLs

https://redmine.teklia.com/issues/10030

We are not using WorkerRun TTLs, and because they are not displayed in the frontend they are causing a lot of confusion for users. We will undo the WorkerRun part of #1871 (closed):

• Drop the WorkerRun.ttl model field
• Remove it from the WorkerRun Django admin
• Remove it from ListWorkerRuns, CreateWorkerRun, RetrieveWorkerRun, UpdateWorkerRun, PartialUpdateWorkerRun
• The ProcessBuilder should use the Corpus.applied_maximum_task_ttl directly to set TTLs on tasks
• Have fun updating all the tests.

changed milestone to %Arkindex 1.7.3

added P2 label

assigned to @erouchet

mentioned in merge request !2521 (merged)

mentioned in merge request frontend!1843 (merged)

closed with merge request !2521 (merged)