There was a datetime bug in the query which resulted in files not being sent to the postal provider.
The trigger-letter-pdfs-for-day task is no longer needed, so rather than fix the query just call collate_letter_pdfs_for_day directly.
Less code is always better.
Deployment considerations: I realized this is strictly not backwards compatible if the scheduled job is in progress and a task is on the queue that no longer exists. This is ok since we will deploy this well before 17:50.