validation: Pass in chainman to UnloadBlockIndex
Summary:
This is a backport of core#19927 [2/6]
https://github.com/bitcoin/bitcoin/pull/19927/commits/74f73c783d46b012f375d819e2cd09c792820cd5
Depends on D9820
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9821