Data backfills | Code cleanup

[sharonwarner]

As we finish our data backfills, we can update our code to expect the data to be in the right state.

1. In schedule_hearing_task.rb, we can update find_or_create_if_eligible to just find the existing ScheduleHearingTask
2. In tasks_controller.rb, we can remove find_or_create_if_eligible from the for_appeal method
3. In hold_hearing_task.rb, we can remove the type check on the task's parent in create_hold_hearing_task!
4. In appeal_repository.rb, we can update create_schedule_hearing_tasks to always create a new HearingTask and a new ScheduleHearingTask
5. In ScheduleHearingTask and DispositionTask (formerly HoldHearingTask), we can add validation such that the parent task always has to be a HearingTask