Drop daily_sorted_letter table

[klssmith]

We stopped writing to this table when we started receiving letter delivery status via HTTPS callback instead of via response files in S3. However, since 2023 we've been using the ft_billing_letter_despatch table for letter billing and to store more detailed information about which letters were classed as sorted vs unsorted so this data is not needed.

[klssmith]

Squawk warnings are fine:

note: Deleting an index blocks selects, inserts, updates, and deletes on the index's table.
  help: Delete the index CONCURRENTLY.

note: Dropping a table may break existing clients.

Nothing is using the table, so breaking existing clients or blocking writes to it while the index is dropped isn't a concern