Consider removing hive_payments table code

Motivation: While reviewing performance of hivemind, I found that hive_payments_to and hive_payments_from indexes are unused. Further searching of the codebase suggests that the hive_payments table is only written to, but never read. It is written to in hive/index/payments.py and the comments suggest this code is related to "promoted posts", something that is no longer possible to my knowledge (presumably it was forked out at some point).

Review if this is really useless code, and remove it if so.