Pass mempool pointer to UnloadBlockIndex

Summary:
PR description:
Instead of relying on the implicit mempool global, pass a mempool pointer (which can be 0). This helps with testing, code clarity and unlocks the features described in #19556

This is a backport of core#19604 [2/3]
https://github.com/bitcoin/bitcoin/pull/19604/commits/fac674db200e6b2d5b32069335fb24e713d7b69f

Depends on D9756

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9757