Log activity on all periodic Celery tasks

As stated in the comment, this would have been helpful during an
incident to give further reassurance that a task had at least
started running - at the time the only evidence for this was the
Cronitor dashboard itself, which we don't often look at.

I've removed other, equivalent "starting" logs, but kept those
that provide additional information in the log message.

app/celery/letters_pdf_tasks.py

@@ -126,7 +126,6 @@ def collate_letter_pdfs_to_be_sent():
     that have not yet been sent.
     If run after midnight, it will collect up letters created before 5:30pm the day before.
     """
-    current_app.logger.info("starting collate-letter-pdfs-to-be-sent")
     print_run_date = convert_utc_to_bst(datetime.utcnow())
     if print_run_date.time() < LETTER_PROCESSING_DEADLINE:
         print_run_date = print_run_date - timedelta(days=1)