db: Remove obsolete methods from CBlockTreeDB.

Summary:
backported first commit (9b0ec1a7f) from PR13243

Depends on D2924

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Maniphest Tasks: T630

Differential Revision: https://reviews.bitcoinabc.org/D2933