walletdb: Remove BerkeleyBatch friend class from BerkeleyDatabase

Summary:
This is a backport of core#19335 [5/5]
https://github.com/bitcoin/bitcoin/pull/19335/commits/74507ce71eb61105fb3ae8460999099234ca7b8b

Depends on D10037

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D10038