Remove the task_failure signal

With !2428 (merged), when the last task for a worker run finishes, all its Queued and Started activities are marked as Error. This obsoletes a signal receiver that we already had, which updated only Started activities to Error when the task failed, got stopped or had a system error.

The unit tests for this signal were moved to EE since this signal could only be triggered through APIs that only EE agents can use, so the failures only show there.